feat: improve planning agent decomposition

[numnx]

What changed

• Expanded the bundled Planning agent with a repository-grounded coverage and risk inventory.
• Added decomposition rules for contracts, consumers, runtime wiring, lifecycle behavior, migrations, failure paths, documentation, and executable integration evidence.
• Removed fixed 3..8 task-count pressure from the universal planning prompt.
• Added regression coverage for the bundled agent and runtime prompt.
• Updated canonical and public agent documentation.

Why

Broad cross-cutting sprints could be compressed into too few tasks, leaving production consumers, integration wiring, recovery paths, and acceptance coverage for sprint-completion QA to discover. Planning now chooses task count from ownership boundaries and risk while still avoiding task-per-file plans and review-only placeholders.

Validation

• pnpm exec vitest run tests/backend/services/code-ux-default-assets-service.test.ts tests/backend/services/planning-prompt-builder.test.ts tests/backend/services/planning-payload-validator.test.ts — 35 passed.
• pnpm run test:backend -- tests/backend/services/code-ux-default-assets-service.test.ts tests/backend/services/planning-prompt-builder.test.ts tests/backend/services/planning-payload-validator.test.ts — full backend suite passed: 5,483 passed, 7 skipped.
• pnpm run lint — passed.
• pnpm run build — passed.
• pnpm run check:docs-web — blocked by six pre-existing unrelated generated public-doc differences; the planning-agent canonical/public pages in this change are synchronized.

Risk and rollback

Risk is limited to planning prompt behavior: broad plans may contain more tasks when repository evidence justifies them. Revert commit c87998e08 to restore the previous planning guidance.