🐛 Add test plan checklist update to develop prompt (closes #88)

[monkut]

Summary

• Insert a "Update the test plan checklist in the PR description" bullet into DEVELOP_AGENT_PROMPT's "On completion" block (between PR push and issue comment).
• Append a "Test plan update:" section mirroring the read → mark → edit flow already in REVIEWPR_AGENT_PROMPT, with explicit carveouts for manual/external verification items and a silent no-op when no ## Test plan section exists.
• Add TestDevelopPromptTestPlanUpdate to tests/test_askcc.py (between TestDevelopPromptTddContent and TestReviewprPromptMergeGuard) with literal-substring assertions on the new wording, mirroring the TestReviewprPromptMergeGuard precedent.

Key Flows

flowchart TD
    A[develop completes] --> B[run /simplify or /refactor]
    B --> C[commit & push branch]
    C --> D[open PR linked to issue]
    D --> E{Test plan section present?}
    E -- no --> G[skip silently]
    E -- yes --> F[mark satisfied items - x , leave manual items - blank]
    F --> H[gh pr edit --body updated]
    G --> I[add issue comment summary]
    H --> I

Loading

Verification

• uv run pytest — passed (192 tests)
• uv run ruff check — passed (no issues)
• uv run pyright — passed (0 errors, 0 warnings)

Test plan

• DEVELOP_AGENT_PROMPT instructs the agent to update test plan items in the PR description after PR creation
• Instruction explicitly says to leave items requiring manual/external verification unchecked
• Instruction is a no-op when no ## Test plan section exists
• Unit tests assert the new instruction strings are present in DEVELOP_AGENT_PROMPT
• uv run pytest, uv run ruff check, and uv run pyright all pass