learnings: audit fixes — preamble in TOC, server-side line-boundary truncation, empty-repo intro (#743)

* learnings: surface preamble in TOC, mirror line-boundary truncation server-side, fix empty-repo intro copy

three audit fixes on top of the recent learnings overhaul (#717):

- `parseLearningsHeadings` now prepends a synthetic `(preamble)` entry
  when a body has non-whitespace content before the first heading. the
  prompt instructs the agent NOT to slurp the whole file when a TOC is
  present, so without this any preamble lines were silently invisible
  (realistic transitional case: an agent partially restructures a
  legacy free-text body and leaves bullets above the first `## `).

- server-side PATCH route now applies the same line-boundary-aware
  truncation as the action (defense in depth via a shared
  `truncateAtLineBoundary` + `MAX_LEARNINGS_LENGTH` exported from
  `action/internal`). the raw `.slice` it used before could leave a
  mid-heading tail on any caller that bypassed the client-side
  truncate, breaking the next-seed TOC parse. removes the duplicated
  cap constant.

- `buildLearningsSection` intro no longer asserts "accumulated by
  previous agent runs" — false for fresh repos with zero history. new
  copy is tense-neutral and works for empty + populated bodies. also
  nudges the agent to re-read after mid-run edits (the inlined TOC
  ranges are a run-start snapshot).

Co-authored-by: Cursor <cursoragent@cursor.com>

* learnings prompt: tighten to single evergreen test, allow tool-quirk bullets when they prevent repeat waste

The blanket "no pullfrog tool quirks" ban was wrong — if the agent burned
calls discovering a quirk this run, recording the workaround prevents the
next run from repeating the waste. Reframe around one litmus ("would a
future run do its work better because this bullet exists?") and trust it
to subsume the scattered don'ts. Drop the 3+ months timeframe (arbitrary)
and the four-example pullfrog/PR/date/play-by-play list (the rule
underneath is "don't anchor facts to repo state that will move"). Cuts
~10 lines from a prompt the model was already mostly ignoring; the
remaining anchor list is narrower and more enforceable.

* audit-learnings-r2: align wiki + tighten re-read nudge

- wiki/prompt.md described the post-run reflection prompt as "bans pullfrog-tool quirks (those belong in tool descriptions, not per-repo learnings), bans PR/review/commit/date references" — that's stale after the prompt rewrite. update to: single-litmus framing, expanded anchor list (now includes version pins + line numbers), and explicit allowance for tool-quirk workarounds when discovery burned calls.
- buildLearningsSection re-read nudge said "re-read after editing" which can be read as "re-read the section you edited". in fact any edit shifts the line numbers of every later section in the TOC, not just the edited one. tighten to make that explicit. mirror the new wording in the wiki example block. update the test substring assertion accordingly.

* postRun: refresh JSDoc to match the reflection prompt rewrite

`buildLearningsReflectionPrompt`'s JSDoc still listed "PR-/review-/commit-/date-anchored facts" and "rediscovery of pullfrog-tool quirks" as failure modes the prompt pushes back on. after b586b4f8 the prompt no longer bans tool-quirk bullets (it explicitly allows them when the agent burned calls discovering the quirk), and the anchor list expanded to cover branch refs, version pins, and line numbers too. update the JSDoc so it describes the prompt that actually exists, and call out the cross-repo drift tradeoff that comes with allowing tool-quirk bullets.

* fix(mcp/issueEvents): narrow event.event before Set.has lookup

octokit's listEventsForTimeline union includes timeline-event members where `event` is `event?: string`. `("event" in event)` does not narrow that property to non-undefined, so `relevantEventTypes.has(event.event)` was passing `string | undefined` to a `Set<string>.has`. typescript only flagged this once `cf-worker-indexing` started seeing the file via the type graph that now reaches mcp through the new `truncateAtLineBoundary` re-export in `action/internal/index.ts`. fix the latent bug at the source: require `typeof event.event === "string"` before the Set lookup.

* learnings: split truncation helpers into MCP-free module

re-exporting `truncateAtLineBoundary` + `MAX_LEARNINGS_LENGTH` from `action/utils/learnings.ts` through `action/internal/index.ts` accidentally pulled the entire MCP type graph into the SDK barrel: `learnings.ts` imports `ToolContext` from `mcp/server.ts`, which transitively wires every tool module under `action/mcp/` into anything that imports from `pullfrog/internal`. for `cf-worker-indexing/tsconfig.json` (`customConditions: ["@pullfrog/source"]`) and the root `tsc` (which compiles the proprietary app routes that import from `pullfrog/internal`), this expanded the type-checked surface and surfaced two latent issues in unrelated files (`mcp/issueEvents.ts`, `utils/subprocess.ts`). a 6-line pure string helper has no business dragging mcp/server.ts into anyone else's type graph.

move both symbols to `action/utils/learningsTruncate.ts`. `learnings.ts` re-exports them so existing callers keep working; `internal/index.ts` re-exports from the truncate-only module so the SDK barrel stays MCP-free.

---------

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: Colin McDonnell <colinmcd94@gmail.com>

utils/learnings.ts

@@ -3,6 +3,9 @@ import { dirname, join } from "node:path";
 import type { ToolContext } from "../mcp/server.ts";
 import { apiFetch } from "./apiFetch.ts";
 import { log } from "./cli.ts";
+import { MAX_LEARNINGS_LENGTH, truncateAtLineBoundary } from "./learningsTruncate.ts";
+
+export { MAX_LEARNINGS_LENGTH, truncateAtLineBoundary };
 
 /**
  * Repo-level learnings — operational facts about a repo (setup steps, test
@@ -34,15 +37,6 @@ import { log } from "./cli.ts";
 
 export const LEARNINGS_FILE_NAME = "pullfrog-learnings.md";
 
-/** server-side cap mirrors `MAX_LEARNINGS_LENGTH` in
- * `app/api/repo/[owner]/[repo]/learnings/route.ts`. truncating client-side
- * keeps the PATCH from being rejected with a 400. raised from 10k → 100k
- * once the TOC affordance landed: with line-range reads via the
- * server-parsed TOC the agent doesn't ingest the whole file, so the cap
- * can grow to whatever curation discipline allows. 100k holds ~400-500
- * short bullets. */
-const MAX_LEARNINGS_LENGTH = 100_000;
-
 export function learningsFilePath(tmpdir: string): string {
   return join(tmpdir, LEARNINGS_FILE_NAME);
 }
@@ -62,23 +56,6 @@ export async function seedLearningsFile(params: {
   return path;
 }
 
-/** truncate at the last newline boundary before `cap` so we don't leave
- * a partial line at the tail (a half-truncated `## Headi` confuses the
- * server's next-seed TOC parse and shrinks visible structure). falls
- * back to a hard `slice` when the line boundary would discard a large
- * run of content — i.e. when the tail of `head` is one giant line (rare:
- * minified pastes, fenced log dumps). losing a partial last line is
- * preferable to losing kilobytes of body. */
-const TRUNCATION_LINE_BOUNDARY_TOLERANCE = 4096;
-function truncateAtLineBoundary(body: string, cap: number): string {
-  if (body.length <= cap) return body;
-  const head = body.slice(0, cap);
-  const lastNewline = head.lastIndexOf("\n");
-  if (lastNewline <= 0) return head;
-  if (cap - lastNewline > TRUNCATION_LINE_BOUNDARY_TOLERANCE) return head;
-  return head.slice(0, lastNewline);
-}
-
 /** read the agent-edited learnings file. returns null when the file is
  * missing or unreadable (treated as "no change"). caps content at the
  * server's max length to avoid a 400 round-trip. */