feat(api): expose open-PR pressure strategy for contributors

[jaso0n0818]

Summary

• Adds GET /v1/extension/contributors/:login/open-pr-pressure — self-scoped, public-safe strategy simulation (open new work / wait / cleanup first) built from cached queue + role context.
• Adds MCP tool gittensory_simulate_open_pr_pressure and planning prompt gittensory_plan_open_pr_strategy for agent-native pre-submission guidance.
• Reuses the feat(scoring): simulate open-pr pressure scenarios #348 simulateOpenPrPressure engine via new open-pr-pressure-response + extension-open-pr-pressure shaping layers with forbidden-term redaction.

Test plan

• npx vitest run test/unit/open-pr-pressure-response.test.ts test/unit/extension-open-pr-pressure.test.ts
• npx vitest run test/unit/mcp-output-schemas.test.ts -t simulate_open
• npx vitest run test/integration/api.test.ts -t "contributor extension context"
• Verified no competing open PR for this surface (original feat(scoring): simulate open-pr pressure scenarios #348 merged the simulator only; no API/MCP exposure yet)
• Null-safe MCP handler returns extension-shaped payload; private terms redacted in scenario text

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.66%. Comparing base (a82f346) to head (33ab44f).

Files with missing lines	Patch %	Lines
src/api/routes.ts	81.25%	0 Missing and 3 partials ⚠️
src/mcp/server.ts	78.57%	2 Missing and 1 partial ⚠️
src/signals/extension-open-pr-pressure.ts	83.33%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1187      +/-   ##
==========================================
- Coverage   94.68%   94.66%   -0.03%     
==========================================
  Files         157      159       +2     
  Lines       19074    19123      +49     
  Branches     6904     6915      +11     
==========================================
+ Hits        18061    18103      +42     
- Misses        424      426       +2     
- Partials      589      594       +5     

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[JSONbored]

failing CI (missing test coverage), closing - also merge conflicts.