feat(01KW5W17): writeRetroProposal tool description matches the schema's accepted proposal types (#404)

[jackmcintyre]

What changed

writeRetroProposal tool description matches the schema's accepted proposal types (#404)

This change delivers the following acceptance criteria:

• AC1: Given the writeRetroProposal tool description and JSDoc, When they are rendered, Then they list exactly the types in RETRO_PROPOSAL_TYPES, derived from the constant rather than a hardcoded count.
• AC2: Given every proposal type in RETRO_PROPOSAL_TYPES, When it is round-tripped through the schema validator, Then each type validates successfully - including the types added since the original seven.

Why

As a retro-analyst, I want the writeRetroProposal description to enumerate every proposal type the schema actually accepts, so that the contract is not misread as silently dropping newer types (persona-append, lesson-consolidation, shared-skill-promotion, etc.).

How to check it yourself

1. Open plugins/flow/mcp-server/src/tools/register.ts and find the writeRetroProposal registerTool call; confirm its description is now the WRITE_RETRO_PROPOSAL_DESCRIPTION constant, not a hardcoded string.
2. Open plugins/flow/mcp-server/src/tools/write-retro-proposal.ts and read WRITE_RETRO_PROPOSAL_DESCRIPTION; confirm it interpolates RETRO_PROPOSAL_TYPES.length and RETRO_PROPOSAL_TYPES.join(", ").
3. From plugins/flow/mcp-server run: npx vitest run src/tools/tests/write-retro-proposal.test.ts
4. As the reviewer (the end-user check), read the rendered description that the test asserts: it lists all 13 types (rule … shared-skill-promotion) with no "seven types" phrasing, and the AC2 cases show every RETRO_PROPOSAL_TYPES entry validating through parseRetroProposalFile. Confirm all tests pass.

Risk and blast radius

Risk tier: medium

The change is scoped to the files in the diff below. Review the diff to judge its actual blast radius — including any effect on shared state, data schemas, or authentication paths — before approving.

What is explicitly not covered: reviewer verification of this summary's accuracy is handled by a separate companion story.

Evidence

The pre-pull-request build-and-test gate passed before this pull request was opened. No pull request can be opened by the automated flow unless both pnpm build and pnpm test exit 0 in the developer's working directory.

Per-criterion covering checks:

• AC1 → plugins/flow/mcp-server/src/tools/__tests__/write-retro-proposal.test.ts (automated test)
• AC2 → plugins/flow/mcp-server/src/tools/__tests__/write-retro-proposal.test.ts (automated test)

Story: native:01KW5W173M53FMZG7DAWPR121Q
Spec: .flow/native-stories/01KW5W173M53FMZG7DAWPR121Q.md
ACs:

• AC1: Given the writeRetroProposal tool description and JSDoc, When they are rendered, Then they list exactly the types in RETRO_PROPOSAL_TYPES, derived from the constant rather than a hardcoded count.
• AC2: Given every proposal type in RETRO_PROPOSAL_TYPES, When it is round-tripped through the schema validator, Then each type validates successfully - including the types added since the original seven.

Derive the writeRetroProposal tool description from RETRO_PROPOSAL_TYPES so it enumerates every accepted proposal type instead of a stale hardcoded seven

Closes #404