learnings: edit-in-place tmpfile (drop update_learnings tool) (#635)

* learnings: edit-in-place tmpfile (drop update_learnings tool)

learnings now follow the PR-summary file pattern: server seeds
`pullfrog-learnings.md` from `Repo.learnings` at startup, agent reads
it as part of context, may edit in place during the post-run reflection
turn, server reads back at end-of-run and PATCHes if changed.

motivation: `update_learnings` required the agent to pass the FULL
merged list as a string parameter — an output-token tax that grew
linearly with the learnings size, and a constant prompt-context
expansion since the contents were also inlined into the LEARNINGS
section. for repos with mature learnings the prompt was getting
visibly noisy in CI logs.

key changes:
- new `action/utils/learnings.ts` (seed/read helpers + 10k cap)
- `main.ts`: always seed; `persistLearnings` mirrors `persistSummary`
  (success path, error path, exit-signal handler, idempotent guard,
  byte-trim equality skip); forwards `model` for `LearningsRevision.model`
- `LEARNINGS` prompt section now contains only the file path + a
  one-line "read it" instruction (no contents inlined)
- `update_learnings` MCP tool deleted; `action/mcp/learnings.ts` removed
- reflection turn (`buildLearningsReflectionPrompt`) reframed around
  file editing with explicit prune-stale + leave-alone-if-nothing-new
  framing
- `learningsStep` removed from every mode checklist — surface lives only
  in the LEARNINGS prompt section + the reflection turn now

* learnings: harden seed step + refresh stale docs (review feedback)

Three findings from PR review, all implemented:

1. wrap learnings seed in best-effort try/catch (action/main.ts) —
   the always-on seed block ran unconditionally and an unwrapped
   `seedLearningsFile` (mkdir + writeFile) failure (ENOSPC, EACCES,
   hostile sandbox) would unwind into the outer main() catch and flip
   an otherwise-successful run to "❌ Pullfrog failed" before the
   agent even started. asymmetric with `persistLearnings`'s own
   best-effort contract. wrap and log on failure; downstream
   consumers (`persistLearnings`, agent harnesses, `resolveInstructions`)
   already handle `learningsFilePath: undefined` cleanly.

2. refresh wiki/main.md — `resolveInstructions` parameter renamed
   from `learnings` to `learningsFilePath` in this PR; the data-flow
   diagram and the resolver dependency table both still showed the
   pre-refactor signature.

3. drop deleted `learnings.ts` from ROADMAP.md + RESEARCH.md
   "missing MCP tool tests" bullets — `action/mcp/learnings.ts` was
   removed in this PR; the bullets are otherwise still accurate.

mcp/server.ts

@@ -39,7 +39,6 @@ import { GetIssueCommentsTool } from "./issueComments.ts";
 import { GetIssueEventsTool } from "./issueEvents.ts";
 import { IssueInfoTool } from "./issueInfo.ts";
 import { AddLabelsTool } from "./labels.ts";
-import { UpdateLearningsTool } from "./learnings.ts";
 import { SetOutputTool } from "./output.ts";
 import { CreatePullRequestTool, UpdatePullRequestBodyTool } from "./pr.ts";
 import { PullRequestInfoTool } from "./prInfo.ts";
@@ -143,6 +142,21 @@ export interface ToolState {
   // persisted) from redundantly re-running the DB PATCH on the
   // success-then-late-throw path.
   summaryPersistAttempted?: boolean;
+  // absolute path to the rolling repo-level learnings markdown file the
+  // agent reads at startup and may edit at end-of-run. seeded by main.ts
+  // for every run from `Repo.learnings` (empty file when no learnings
+  // exist yet); read back at end-of-run to persist any edits.
+  learningsFilePath?: string;
+  // exact bytes of the seeded learnings file at run start. compared
+  // against the file content at end-of-run to detect "agent never touched
+  // it" — in that case persistLearnings skips the DB PATCH (saving the
+  // identical content would be a no-op write that wastes a LearningsRevision
+  // row and the API round-trip).
+  learningsSeed?: string;
+  // mirror of `summaryPersistAttempted` for the learnings tmpfile — guards
+  // the error-path / exit-signal callers from a redundant second PATCH
+  // after the success path already persisted.
+  learningsPersistAttempted?: boolean;
   output?: string;
   usageEntries: AgentUsage[];
   model?: string | undefined;
@@ -189,8 +203,8 @@ export interface ToolContext {
   tmpdir: string;
   // repo-level OSS flag + account-level billing plan. together they decide
   // whether pullfrog is paying for marginal infra — see isInfraCovered in
-  // utils/runContext.ts. plan gating for things like update_learnings is
-  // enforced server-side via 402, so we pass plan along mostly for future
+  // utils/runContext.ts. plan gating for endpoints like the learnings PATCH
+  // is enforced server-side via 402, so we pass plan along mostly for future
   // use / observability. see wiki/pricing.md.
   oss: boolean;
   plan: AccountPlan;
@@ -288,7 +302,6 @@ function buildOrchestratorTools(ctx: ToolContext, outputSchema?: JsonSchema): To
     DeleteBranchTool(ctx),
     CreatePullRequestTool(ctx),
     UpdatePullRequestBodyTool(ctx),
-    UpdateLearningsTool(ctx),
   ];
 }