feat: add shared Task collaboration and workspace views

[IchenDEV]

Canonical change Artifact

• Change: docs/sdlc/changes/2026-09-01-shared-task-collaboration/change.md
• Related UI changes:
  • docs/sdlc/changes/2026-09-01-taskboard-view-switching/change.md
  • docs/sdlc/changes/2026-09-01-align-nested-task-provenance/change.md
  • docs/sdlc/changes/2026-09-01-default-project-group/change.md
  • docs/sdlc/changes/2026-09-01-empty-heading-project-typography/change.md
  • docs/sdlc/changes/2026-09-01-unify-task-row-style/change.md
• Schema: 2
• Lifecycle status: executing
• Intent/Spec approval: user request and screenshot feedback on 2026-09-01
• Risk and scope: high; additive Core/Server collaboration contract plus Desktop Host bridge, TaskBoard views, and SessionRail hierarchy

Outcome

• Adds the first authoritative two-member Task collaboration loop: Workspace/Member/Device identity, comments, Suggestions, owner approval, revisioned snapshots, Attention projection, idempotent execution commands, and revocation.
• Keeps one Core as the authority and reuses the existing Task → Work Item → Agent → Session execution path.
• Adds List/Board TaskBoard views with shared filtering and selection.
• Makes Project the sidebar grouping boundary and applies one consistent two-line style to ordinary Task rows.
• Rebases cleanly over PR feat(desktop): add state-aware Git next action #217 and preserves its TaskBoard mutation gate.

Verification

• Every AC-N acceptance criterion has owner-run command or linked evidence in the Artifacts.
• The Artifacts record Verdict: and Residual risk: consistently with their current status.
• High-risk verification is independent; the primary Artifact remains executing until that review completes.
• cargo test -p codetwo-core -p codetwo-server
• cargo test -p codetwo-desktop-host
• cargo check --workspace
• Desktop bun test: 860 passed, 0 failed, 5,110 expectations.
• bun run build:renderer
• Documentation, SDLC, worktree, Gate regression, and diff checks.
• User-visible TaskBoard and sidebar changes include native-window and narrow-width evidence in the related Artifacts.

Review and release

• Risk level: high
• Release target: none
• Rollback: revert this additive change; collaboration tables remain inert and no existing data is migrated or deleted.
• Merge approval: pending independent verification and human review
• Release approval: not applicable
• This PR does not infer merge, deployment, or release authority from implementation work.