Add phase 1 implementation plan, refine spec

The plan embeds code that was run in a scratch copy before writing it
down (38 tests, helper stats against the real VM data). Drafting it
settled a few details now reflected in the spec: version also returns
the overlays, an upstream_branch helper key, a shared fmt module and a
launcher, follow resuming on the next jobs poll instead of its own
backoff, and sbpull returning sbshadow's exit status.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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