fix(start-work): honor explicit plan name and strip ultrawork keywords

[KNN-07]

Summary

Fixes the bug where /start-work [PLAN] ultrawork would work on a stale plan instead of the explicitly specified plan.

Problem

When user types /start-work my-plan ultrawork:

1. The auto-slash-command hook captures args as my-plan ultrawork (keyword pollutes plan name)
2. The keyword-detector hook triggers on ultrawork and injects ultrawork-mode
3. The start-work hook ignored the explicit plan name and always resumed the existing boulder.json state

This caused the agent to work on the wrong plan (stale boulder state) instead of the user-specified plan.

Solution

The start-work hook now:

1. Extracts plan name from <user-request> section
2. Strips ultrawork/ulw keywords from the plan name argument
3. Searches for matching plan (exact match first, then partial match)
4. Uses the matched plan instead of blindly resuming stale boulder state
5. Clears stale boulder.json when switching to explicitly requested plan

Test Cases Added

• ✅ Should select explicitly specified plan name, ignoring existing boulder state
• ✅ Should strip ultrawork keyword from plan name argument
• ✅ Should strip ulw keyword from plan name argument
• ✅ Should match plan by partial name

Verification

bun test src/hooks/start-work/
# 12 pass, 0 fail

---

Summary by cubic

Fixes /start-work to honor the explicit plan name and ignore “ultrawork/ulw” keywords, preventing sessions from resuming a stale plan. Users typing “/start-work my-plan ultrawork” now start on “my-plan” as intended.

• Bug Fixes
  • Extracts the plan from the section and strips ultrawork/ulw keywords.
  • Finds plans by exact, then partial match; switches to the matched plan and clears stale boulder state.
  • Handles edge cases: informs when the requested plan is already complete, or lists available incomplete plans when not found.

^{Written for commit e925ed0. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

[code-yeongyu]

@sisyphus-dev-ai review ulw

[kdcokenny]

lgtm