fix(boatstack): stop shipped features re-registering as ambiguous plan candidates

[bigboateng]

Problem

In a repo that has shipped more than one feature and uses worktree delivery mode, next-status from a fresh build worktree returns BLOCKED / AMBIGUOUS "More than one saved feature plan is available", listing every historically shipped feature as a candidate (taxweave: 22 shipped + 1 active = 23). This blocks all forward progress.

Root cause

featurePlanCandidates (next.go) counted a feature dir as an open plan when it had plan.md but no delivery state.json. Delivery state.json is stored per-worktree (deliveryStateDirectory uses git rev-parse --git-dir), and workspace.cleanup_after = merge deletes the feature's worktree on ship — destroying the state.json while the committed plan.md / plan.lock.json / pr.md survive. So every shipped feature matched "planned + no state" and re-registered as an open candidate. ignored_deliveries filters the active-delivery path, not this candidate path.

Fix (Part B only)

The candidate check now skips any feature carrying plan.lock.json (locked/built) or pr.md (shipped) before the state.json test, mirroring orphanedFeatureArtifacts. It relies on the durable committed artifacts, so the loss of an ephemeral state.json on cleanup is harmless. A dir with plan.md only correctly remains the single open candidate.

Part A rejected: relocating delivery state to --git-common-dir would make every active delivery visible in all worktrees, breaking intentional per-worktree isolation (TestManagedDeliveryStateDoesNotBlockUnrelatedWorktrees) and changing cross-worktree push-denial (ActiveManagedDeliveries → ClassifyCommand). No delivery-state locality change is made.

Tests

• TestFeaturePlanCandidatesExcludesLockedAndShippedFeatures — unit.
• TestResolveNextIgnoresShippedFeatureCandidates — reproduction of the taxweave scenario.
• TestResolveNextIgnoresShippedFeaturesFromLinkedWorktree — worktree conformance (the exact reported symptom).
• TestManagedDeliveryStateDoesNotBlockUnrelatedWorktrees — unchanged, still green (isolation not touched).

Verification

From product-engineering-loop: go build ./..., go vet ./..., go test ./... all pass. From labs/12-product-engineering-loop: python3 -m unittest tests.test_product_loop (18 tests) passes.

Rollout (taxweave, after merge)

1. Bump the helper version.
2. Revert PR #304 to restore the 22 shipped feature dirs.
3. Confirm next-status resolves firm-status-finance-aesthetic as the single candidate.