feat: make issue blockers actionable (AXI-186)

[Codename-11]

Summary

• Add a direct issue-detail blocker control backed by native BLOCKED_BY relations, including searchable selection, navigation, removal, and automatic terminal-state clearing.
• Surface active blockers as an issue banner and make the Dashboard Blocked tab use the same canonical dependency predicate as issue filtering and queue exclusion.
• Remove dependency-blocked issues from ordinary dashboard work lanes and keep /blocked <reason> as a separate operator-attention request.

Verification

• pnpm lint
• pnpm typecheck
• pnpm vitest run tests/unit/issue-blocker-control.test.ts tests/unit/dashboard-operator-home.test.ts tests/unit/attention-request-model.test.ts — 9/9 passed
• Focused production-build Playwright flow for set, banner, dashboard reconciliation, deduplication, and removal
• Desktop and mobile in-app browser QA; no console errors
• pnpm ci:local:quality — local Docker Desktop service is stopped and cannot be started from the non-elevated session. Exact-head GitHub CI is the required release gate.