refactor(router): extract webhook handlers into dedicated modules

[aaight]

Summary

Refactors the src/router/index.ts god module (726 lines) by extracting provider-specific webhook handlers and shared utilities into dedicated modules.

• Extracts shared parsing utilities into src/router/webhookParsing.ts (eliminates verbatim duplication of parseGitHubWebhookPayload between router/index.ts and server.ts, and replaces 6x copy-pasted rawHeaders extraction)
• Extracts Trello webhook logic (9 functions + interface) into src/router/trello.ts
• Extracts GitHub webhook logic (5 functions) into src/router/github.ts
• Extracts JIRA webhook logic (3 functions) into src/router/jira.ts
• Slims src/router/index.ts from 726 → ~190 lines (routing/orchestration only)
• Adds 54 new unit tests covering all extracted modules (webhookParsing.test.ts, trello.test.ts, github.test.ts, jira.test.ts)

Test plan

• All 54 new unit tests pass
• Full test suite: only 6 pre-existing failures (unrelated config/projects.test.ts), no regressions
• TypeScript: zero errors (npm run typecheck)
• Lint: zero errors (npm run lint)

Card

https://trello.com/c/p8olsrA4/73-refactor-router-indexts-extract-webhook-handlers-deduplicate

🤖 Generated with Claude Code

[nhopeatall]

🔍 Reviewing PR...

[nhopeatall]

LGTM — clean extraction refactor that breaks a 726-line god module into well-bounded provider-specific modules with proper dependency injection (triggerRegistry as parameter instead of closure capture). Verified behavioral equivalence of extracted code, confirmed deduplication of parseGitHubWebhookPayload between router and server, and all 55 new tests pass with zero TypeScript errors.