Close workspace transition deadlocks

[bigboateng]

Why

Validated planning packages could remain in the source checkout while approval, autonomy, or activation continued in another workspace. This left a reachable workspace-transition deadlock and could bind authority to the wrong branch.

What changed

• Route validated plans through the existing workspace-cut operation before approval or autonomy.
• Create, adopt, or reuse only exact-base, unowned feature destinations.
• Move embedded planning packages transactionally and preserve detached controller identity.
• Return the destination repository, branch, base commit, plan fingerprint, controller mode, and transition outcome.
• Continue later commands from the returned destination path.
• Fail closed on divergence, dirtiness, ownership conflicts, package conflicts, and injected transition failures.
• Remove docs/why-these-steps.md and repoint its maintained references.

Evidence

• go test ./...
• go vet ./...
• go build ./...
• Repository-contract Python tests
• Release-note policy and committed preflight
• Locus nonblockingness: verified, event completeness discharged, frontier target met

Review order

1. boatstack/workspace.go and boatstack/detached.go
2. Routing and authority changes in next.go, readiness.go, autonomy.go, and flow control
3. workspace_transition_conformance_test.go and solution-closure tests
4. Generated instructions, public claim evidence, and release note

Risks and rollback

The change expands workspace-cut from create-only behavior to deterministic create, adopt, or reuse behavior. Every accepted destination must match the fetched base and complete planning-package digest. Reverting this commit restores the previous transition behavior.

Release note

Added release-notes/2026-08-09-workspace-transition-coreachability.md.