[Phase 3] Deprecate idea-enhancer in .github-private: remove workflow + scripts + test, update the pipeline doc

[github-actions]

Story

As a maintainer of the .github-private idea→initiative pipeline,
I want remove idea-enhancer.yml, scripts/idea-enhancer/**, and tests/test_idea_enhancer.bats, drop its lint hook, and update the pipeline doc to describe a single enhancer,
so that there is exactly one Discussion-triggered enhancer in .github-private (no double-enhancement), with the idempotency marker continuity preserved across the cutover.

Acceptance Criteria

1. .github/workflows/idea-enhancer.yml, scripts/idea-enhancer/gather-candidates.sh, scripts/idea-enhancer/post-enhancement.sh, and tests/test_idea_enhancer.bats are removed.
2. .github/workflows/lint.yml no longer references tests/test_idea_enhancer.bats and the lint job still passes.
3. docs/initiatives/idea-to-initiative-pipeline.md is updated: the 'idea-enhancer.yml — enrich human-authored ideas' section and its pipeline-diagram box are replaced by a description of the single enhancer (feature-ideation's enhancement mode), and the marker reference is retained as the canonical idempotency contract.
4. Marker continuity verified: an Idea already carrying <!-- idea-enhancer:enhanced --> is not re-enhanced after the cutover (confirmed against an already-enhanced .github-private Idea, e.g. Enhanced Prompting Frameworks #851 from story 3).
5. Post-cutover single-enhancement verified: opening a fresh human Idea in .github-private produces exactly ONE enhancement comment (not two).
6. A repo-wide grep confirms no remaining references to idea-enhancer / idea_enhancer outside historical docs/changelog.

Tasks / Subtasks

• Delete idea-enhancer.yml, scripts/idea-enhancer/ (both scripts), and tests/test_idea_enhancer.bats. (AC: test issue from agent #1)
• Remove the tests/test_idea_enhancer.bats line from .github/workflows/lint.yml and confirm lint still passes. (AC: Go-live improvements for PR review agent #2)
• Rewrite the idea-enhancer section + pipeline diagram in docs/initiatives/idea-to-initiative-pipeline.md to describe the single (feature-ideation) enhancer; keep the marker as the idempotency contract. (AC: Add @claude delegation, auto-merge, and rebase handling #3)
• Verify marker continuity (already-enhanced idea is a no-op) and post-cutover single-enhancement (fresh idea gets one comment). (AC: Optimize review: small-PR and incremental fast paths #4, feat: add Copilot engine support via REVIEW_ENGINE toggle #5)
• Grep the repo for leftover idea-enhancer references and clean up any stragglers. (AC: feat: feature ideation framework — adversarial proposer/challenger/synthesizer #6)

Dev Notes

• HARD GATE: only deprecate after story 3 proves the folded enhancement mode works on the next ring — removing idea-enhancer before the folded mode is live and validated would leave .github-private with NO human-idea enhancement (re-opening the exact gap Feature Ideation doesn't enhance human ideas #638/feat: implement issue #638 — Feature Ideation doesn't enhance human ideas #644 closed). blocked_by story 3 enforces this.
• Known references to clean up (grepped 2026-06-21): docs/initiatives/idea-to-initiative-pipeline.md (the dedicated section, the ASCII pipeline diagram '★ human opens an Ideas Discussion ──> THIS (enhance comment)' box, the table row, and the line ~67 marker mention — keep the marker, drop the idea-enhancer ownership), and .github/workflows/lint.yml (line ~62 runs the bats test explicitly).
• The idempotency marker is <!-- idea-enhancer:enhanced -->, defined in both scripts/idea-enhancer/{gather-candidates,post-enhancement}.sh and asserted in the bats test. After removal, the marker contract lives in the folded feature-ideation mode (petry-projects/.github, untracked prerequisite) — it MUST keep using the same string so previously-enhanced ideas stay skipped. Do not change or strip the marker text anywhere it is documented.
• idea-enhancer.yml is org-private automation (not a thin-caller stub), so removing it is purely a .github-private change — it is not referenced by any consumer repo. CLAUDE.md's thin-caller list (claude.yml, agent-shield.yml, auto-rebase.yml, dependabot-automerge.yml) does not include it, so no thin-caller guidance is affected.
• Testing standard: after deletion, run the remaining bats suite and shellcheck on scripts/ to confirm nothing else depended on the removed scripts.

Project Structure Notes

Pure deletion + doc/lint cleanup within this repo. No new files. Removes the scripts/idea-enhancer/ directory entirely.

References

• .github/workflows/idea-enhancer.yml
• scripts/idea-enhancer/gather-candidates.sh
• scripts/idea-enhancer/post-enhancement.sh
• tests/test_idea_enhancer.bats
• .github/workflows/lint.yml (runs the bats test)
• docs/initiatives/idea-to-initiative-pipeline.md (idea-enhancer section)

Likely target surface

• .github/workflows/idea-enhancer.yml
• scripts/idea-enhancer/
• tests/test_idea_enhancer.bats
• .github/workflows/lint.yml
• docs/initiatives/idea-to-initiative-pipeline.md

Story prepared by the BMAD Scrum Master (Bob) for epic #872. Status: ready-for-dev.