feat(mcp): add task mutation tools

[pablojaku]

Summary

• add create_task, update_task, and complete_task MCP tools behind tasks:write
• extract user-scoped task mutation helpers so UI actions and MCP handlers share ownership and completion rules
• validate defaults, nullable fields, future due dates, activity-type ownership, and no-op updates
• add real Streamable HTTP E2E coverage plus MCP routing evaluations and documentation

Implementation notes

• Activity-type labels are matched case-insensitively per user and created when absent.
• Plain-text descriptions are escaped into minimal Tiptap-compatible HTML and serialized back as plain text.
• Explicit past due dates are rejected, while unrelated updates to overdue tasks remain valid.
• Setting status to complete without a percentage sets 100%; complete_task updates both fields atomically.
• Deletion and activity timers remain out of scope.

Verification

• npm run lint
• npx tsc --noEmit --incremental false
• targeted Vitest: 6 files, 81 tests passed
• full Vitest: 240 files, 2,895 tests passed
• npm run build
• full Chromium E2E: 91/93 passed in the four-worker run; the two unrelated add-job timeouts both passed when rerun serially (2/2)
• the real-transport MCP task workflow passed in the full Chromium run
• local MCP routing evals: 13/13 passed

Dependency

This is the second of two focused PRs and is stacked on #120. Once #120 merges, this PR's diff will contain only the mutation work.

Closes #119