feat(desktop): add Work Board Phase 3 start-task spike

[somewan820]

Summary

Implements the thin Work Board Phase 3 validation spike for #2560.

• Adds a default-off Start task path for project-scoped Work Board items whose target resolves to one available, non-archived Host/project target.
• Reuses the existing New Task and Composer first-send flow; the draft contains the item title and optional notes.
• Persists an idempotent Host-scoped linkedSessions reference only after a projected first-send outcome.
• Navigates linked items through the existing Desktop Session identity path.
• Keeps Inbox target selection, execution-state projection, automatic completion, model-visible tools, turn-tail injection, result refs, prioritization, and board redesign out of scope.
• Clears pending Work Board claims on refused, failed, cancelled, or unknown first-send outcomes so retries are not permanently blocked.

The experiment is enabled only in a development renderer with:

VITE_MAKA_WORK_BOARD_START_TASK=1

This is intentionally not a persisted user setting. The goal is maintainer/contributor dogfooding before deciding whether to productize the entry and continue later Work Board phases.

Verification

• npm test: 1580 passed, 12 skipped; runtime-host and other relevant suites passed.
• npm --workspace @maka/desktop run build:main: passed.
• Focused first-send cleanup tests: 16 passed.
• Renderer TypeScript check: passed.
• Biome lint: passed.
• git diff --check: passed.

The storage workspace test run remains affected by the existing sandbox EPERM chmod restriction under ~/Library/Application Support/Maka / ~/Library/Caches/Maka; this is environmental and unrelated to the changed Work Board behavior.

Generated-by: Codex

AI use

Codex assisted with implementation, review, and test execution. The contributor reviewed the resulting changes and owns the final PR.