docs: scope M5 to dry-run-first slice with exercised-vs-extension posture

[aryeko]

Records the M5 scoping decision and sharpens the milestone.

• Adds an Approach field with the M5a (design slice) / M5b (implementation slice) split and a per-seam posture table (exercised vs named extension point).
• Flags entry criterion 3 (Jig design names the first local execution host + minimum policy posture) as currently unmet; M5a closes it.
• Sharpens exit criteria to dry-run-first under policy with no privileged actions, and requires the authorization/fence to be invoked and emit the requested -> authorized/denied -> runner-owned record chain even in dry-run.
• Clarifies the machine-readable plan instance is M5's own artifact (M4 handed off only the shape).
• Adds a kill-assumption against unexercised no-op code stubs for non-traversed seams.

Docs-only; pnpm check green.