feat(sprint-2): implement issue detail page, claims engine, and contribution workspace

[ashishsinghbora]

Sprint 2 Summary

• Issue Claims Engine (src/lib/issues/claims.ts):
  • 7-day automatic claim reservation with expireOverdueClaims engine.
  • Progression level limits: 1 active claim for Level 1/2 beginners; up to 2 active claims for Level 3+ builders.
  • Conflict prevention: rejects concurrent claims on the same open issue while honoring self-claims.
  • Release operation allowing contributors or maintainers to release claims early without penalty.
  • Unit and integration tests in src/lib/issues/claims.test.ts verifying all scenarios (6 new tests, 42/42 passing platform-wide).
• Issue Detail & Contribution Workspace Page (/issues/[id]):
  • Displays full issue description, repository, labels, difficulty, effort estimation, and quality/merge criteria.
  • Dynamic Claim Status Panel:
    • Unauthenticated: prompt to sign in with GitHub.
    • Available: one-click claim with quota checking.
    • Claimed by other: contributor avatar, username, and remaining days.
    • Claimed by user: active workspace state with step-by-step contribution checklist (git commands, copy helpers, test instructions, and PR template Fixes #ID).
• Claim & Release API Endpoints:
  • POST /api/issues/[id]/claim
  • POST /api/issues/[id]/release
• Marketplace & Dashboard Integration:
  • Updated issue cards in /issues to route to /issues/[id] with "Start Contribution" CTA.
  • Auto-expiration of overdue claims on dashboard load.