docs: require engineering apprenticeship workflow

[avery-salhotra]

What changed

• add a tracked repository policy requiring the engineering-apprenticeship workflow for software work
• preserve proportional exceptions for conversation, status, navigation, explanation, research-only work, and urgent containment
• document the four progressive stages, four human checkpoints, and human-owned final acceptance
• document that the reusable skill remains local and Git-ignored
• explain that fresh clones receive the policy but must install or recreate the local skill

Why

The reusable skill implementation is intentionally local, but the repository still needs a durable, shared policy describing how software work must be performed. This carries the accurate information out of the older worktree without copying its stale explicit-invocation wording.

Validation

• focused policy and file-scope checks passed
• npm run verify passed with 26/26 tests, TypeScript, production build, and dependency guard
• git diff --check passed
• independent implementer, blind-review, alignment, and human review completed