feat(boatstack): context-aware capability provisioning + registration [3/3]

[bigboateng]

Slice 3 of 3 — Context-aware capability provisioning + registration

Stacked on Slice 2 (boatstack/capture-evidence-orchestration). Review/merge Slices 1 and 2 first.

Closes the loop for repositories that cannot yet produce a capability's evidence: instead of silently leaving a gap, Boatstack detects the stack and guides setup — without ever imposing a framework.

What changes

• detectFrontendStack (modeled on detectTestCommand): reads package.json / lockfiles / config to report framework, package manager, dev command, and Playwright/Cypress/Storybook presence. Reports none honestly for backend-only repos.
• provision-capability: returns a tiered guide — available (command already resolves) / provision (stack-tailored steps + the capture contract) / unsupported (no frontend; never forced).
• capability-register: records the repository-owned command, mutating the canonical .boatstack-project.json source and regenerating the full export so both stay in sync — or round-tripping the generated project.json alone when there is no source.
• Auto-plan now surfaces a material provisioning decision ([1a] provision now as its own slice / [1b] bundle the harness into the feature slice / [1c] record the gap and defer) when a scenario is relevant but no command resolves. A surfaced choice, never an imposed step.

Constraints held

• Boatstack ships the contract + guide; the harness is authored in the user's repo.
• No external-lab imports; config writes go through the atomic-write + Generated/MarshalJSON idiom.

Verification

• go test ./... green (detection, guide tiers, source/export sync + fallback registration).
• Python projection suite 18/18 (auto-plan text edit tolerated — substring-based).
• Release note included.