[Bugfix #676] Scope porch check artifact resolution to the worktree

[waleedkadous]

Fixes #676.

Summary

• porch approve plan-approval failed with Plan not found when the plan lived in a builder worktree. findStatusPath (PR Decouple worktree/branch/PR, add verify phase, remove TICK (#653) #674) already resolves the worktree, but the artifact resolver and the cwd passed to runPhaseChecks remained rooted at process.cwd(), so plan_exists looked in the main tree instead of the worktree.
• check, done, and approve now derive the artifact root from the resolved status path via a new getArtifactRoot helper and rebuild a LocalResolver scoped to that path. runPhaseChecks is also invoked with the worktree as its cwd, so shell-based checks resolve relative paths against the correct tree.
• Confirmed existing behavior: checks still run before any state write in approve; a failed check cannot flip the gate to approved.

Changes

File	What
state.ts	Add getArtifactRoot(statusPath) helper (three levels up from status.yaml).
artifacts.ts	getResolver(workspaceRoot, artifactRoot?) lets callers point the local backend at a worktree while still loading config from the main workspace.
index.ts	check/done/approve build a scoped resolver and pass artifactRoot to runPhaseChecks and resolveArtifactBaseName.
__tests__/bugfix-676-approve-worktree-artifacts.test.ts	Regression coverage for approve, check, and done against a worktree layout (including a negative test that a decoy plan in the main tree cannot approve the gate).

Test plan

• pnpm exec vitest run src/commands/porch/__tests__/bugfix-676-approve-worktree-artifacts.test.ts — 4/4 pass.
• pnpm exec vitest run src/commands/porch — full porch suite 275/275 pass.
• Verified the new tests fail on main without the fix (3/4 fail exactly where expected).
• pnpm exec tsc --noEmit clean.
• Manual repro: architect runs porch approve NNN plan-approval --a-human-explicitly-approved-this from repo root against a plan that lives in .builders/<slug>/codev/plans/ and the gate flips to approved.

Pre-existing failing tests in adopt.test.ts, consult.test.ts, update.test.ts, and session-manager.test.ts are unrelated to this change.

[waleedkadous]

Architect review: Approved. Worktree-scoped artifact resolution with 4 regression tests, CI green.