feat(desktop): make tasks a session workspace

[IchenDEV]

Canonical change Artifact

• Change: docs/sdlc/changes/2026-09-01-task-session-workspace/change.md
• Schema: 2
• Lifecycle status: verified
• Intent/Spec approval: user-approved prototype option A and direct implementation/lint follow-ups on 2026-09-01
• Risk and scope: medium; desktop TaskBoard workspace, integration wiring, focused tests, lint/mutation gates, and canonical Artifact

Outcome

TaskBoard is now a dense Task-to-Session workspace instead of a four-column Kanban surface. Tasks retain their existing editing and lifecycle actions, expand into newest-first Session history, and drive a persistent inspector that shows the selected Session's activity, checkout/worktree, and best-effort pull request.

The implementation preserves Task persistence and Session/Core authority, scopes pull requests to each Session checkout, and keeps lookup failures quiet. The former 1,268-line page is split into responsibility-owned modules. ESLint automatically covers new TaskBoard TypeScript modules with useful complexity, file-size, unused-code, local-redundancy, and unsafe-any rules; Stryker remains the separate deterministic model test.

Rendered evidence:

• Desktop light
• Desktop dark
• Narrow drawer

Verification

• Every AC-N acceptance criterion is mapped to one actual command or linked evidence item.
• The Artifact records Verdict: verified and a concrete residual risk.
• Verification mode, verifier, and date match the medium-risk owner-verification lane.
• Post-merge desktop suite: 826 tests across 142 files, 4,908 assertions, zero failures.
• bun run build:renderer passed after lint, Stylelint, TypeScript, and Vite production build.
• bun run mutation:taskboard: 86 valid mutants killed, 0 survived, 0 without coverage; 84 compile-invalid mutants rejected.
• Documentation catalog, SDLC plain/worktree checks, Gate self-tests, and git diff --check passed.
• Real light, dark, and narrow evidence is linked above.

Review and release

• Risk level: medium
• Release target: none
• Rollback: revert the TaskBoard workspace commits; Task and Session persistence formats are unchanged.
• Merge approval: pending human review
• Release approval: not applicable
• This PR does not infer merge, deployment, or release authority from implementation work.