Make task dispatch atomic and idempotent

[iqrahadian]

External task producers can now assign and start a task without racing renderer triage or launching duplicate agent sessions. Agent startup context is also reduced to instructions relevant to the selected backend and task.

• Persist agent_id and skill_ids atomically during task creation
• Share one in-flight session start per task and reuse an existing healthy session
• Respect disabled auto-start on recurring task instances
• Generate only AGENTS.md or CLAUDE.md for the selected backend and remove volatile timestamps
• Add heartbeat and task-orchestration instructions only when the task requires them
• Add regression coverage for atomic assignment, duplicate starts, recurrence opt-out, and prompt generation

Validation:

• 149 test files passed; 2,271 tests passed and 2 skipped
• pnpm typecheck passed
• pnpm build passed
• pnpm lint passed with zero errors; existing warnings remain

Risk/rollback:

• Changes task creation and session-start behavior for all callers. Reverting this commit restores the previous renderer-driven assignment and startup flow.