feat: work item dependency graph with blocked status and notifications

[DorianZheng]

What changed

• added dependsOn support plus shared dependency utilities for blocked-state checks, unresolved dependency summaries, and circular dependency detection
• enforced dependency-aware move guards in both the renderer store and the main-process agent action validators so blocked items cannot move to active
• added dependency add/remove actions, blocked indicators, dependency editing in the item inspector, automatic unblocked-assignee notifications, and a project-level dependency graph view
• fixed follow-on renderer/runtime regressions uncovered during verification, including external-channel compose disabling, runtime bridge snapshot compatibility, and the full-suite Vitest timeout budget

Why

Work items need explicit prerequisite tracking so execution cannot start before upstream work is complete, and assignees should be notified as soon as blocked work becomes actionable again.

Validation

• pnpm run typecheck
• pnpm test

Notes

• branch: feat/work-item-dependency-graph
• commit: 26ec9f3

[DorianZheng]

Superseded by #90 after the original head branch picked up an unrelated follow-up commit.