Align Plan-mode prompts on report_progress as the canonical plan tool (#786)

* fix: align Plan-mode prompts on report_progress as the canonical plan tool

Fixes #673.

Three sites disagreed on where Plan output should be posted, letting a
model synthesize a broken third interpretation (initial post via
`report_progress({ target_plan_comment: true })`, which then misses the
`existingPlanCommentId` precondition). This PR aligns all three on
`report_progress` as canonical, with `target_plan_comment` reserved for
revisions only:

- `action/modes.ts` Plan step 4 — spell out that the initial plan post
  uses `report_progress` WITHOUT `target_plan_comment`, and that
  revisions go through `select_mode`'s PlanEdit override.
- `action/mcp/comment.ts` `target_plan_comment` flag description —
  make the "revisions only" precondition explicit and call out the
  initial-post path by name.
- `action/utils/instructions.ts` Progress reporting paragraph — drop
  the misleading "(e.g., Plan comments)" parenthetical that read as
  "use create_issue_comment for plans".

`PlanEdit` (in `action/mcp/selectMode.ts`) was already correct and is
unchanged.

Intentionally out of scope (to keep the fix minimal): a `publish_plan`
tool, removing the vestigial `create_issue_comment({ type: "Plan" })`
branch, hardening the run-end cleanup guard for the
`target_plan_comment but no existingPlanCommentId` fallthrough, and
renaming `target_plan_comment`.

* align create_issue_comment description with report_progress as canonical plan tool

mcp/comment.ts

@@ -68,7 +68,7 @@ export function CreateCommentTool(ctx: ToolContext) {
     description:
       "Create a comment on a GitHub issue or PR. " +
       'Example: `create_issue_comment({ issueNumber: 1234, body: "Thanks for the report." })`. ' +
-      "For progress/plan updates on the current run use report_progress instead. Use type: 'Plan' for plan comments.",
+      "For progress/plan updates on the current run use report_progress instead — plan output (initial post AND revisions) is always posted via report_progress, never via this tool.",
     parameters: Comment,
     execute: execute(async ({ issueNumber, body, type: commentType }) => {
       const bodyWithFooter = addFooter(ctx, body);
@@ -153,7 +153,7 @@ export function EditCommentTool(ctx: ToolContext) {
 export const ReportProgress = type({
   body: type.string.describe("the progress update content to share"),
   "target_plan_comment?": type("boolean").describe(
-    "when true, update the existing plan comment (from select_mode lookup) instead of the progress comment; use when editing an existing plan"
+    "for revising an existing plan comment ONLY. set to true only when the PlanEdit checklist from select_mode tells you to (i.e. a prior plan comment was found for this issue). NEVER set on the initial plan post — the initial plan reuses the run's progress comment and is posted by calling report_progress without this flag."
   ),
 });