Refactor CLI analyze to use pkg/prism (Phase 2)

[hidetzu]

Summary

CLI analyze now calls pkg/prism.Analyze() internally. The CLI JSON output is byte-identical to pkg/prism.Result, verified by golden tests. This completes Phase 2 of ADR-0002 and ensures CLI/library symmetry.

Breaking changes (CLI JSON output, v0.2 → v0.3)

Field	v0.2.x	v0.3.0
`provider`	top-level	`pull_request.provider`
`pull_request.description`	included	removed (use `prism fetch`)
`pull_request.url`	not present	added
`changed_files[].patch`	included	excluded by default
Empty fields	always present	omitted (`omitempty`)

These changes match the spec defined in ADR-0002 and are documented in `docs/json-schema.md`.

What's in

Code

• Delete: `internal/usecase/analyze.go` and tests (replaced by `pkg/prism.Analyze` + formatter)
• Delete: `internal/formatter/types.go` (formatter now uses `prism.Result` directly)
• Rewrite: `internal/formatter/{json,markdown,text}.go` to accept `prism.Result`
• Update: `cmd/prism/main.go runAnalyze` to call `pkg/prism.Analyze` and route through formatter
• Update: `cmd/prism` exit-code mapping to also recognize `prism.Err*` sentinels
• Add: `provider.NewRegistryWithGitHubBaseURL` test seam (used by pkg/prism integration tests)
• Add: `pkg/prism/integration_test.go` with 5 happy-path tests using a mock GitHub server
• Update: golden test fixture to new JSON shape

Docs

• `README.md`: JSON/Markdown examples updated, Roadmap reorganized (v0.3.0 = library API, v0.4.0 = policy files, v0.5.0+ = SARIF/IDE)
• `docs/adr/0002-public-api-boundary.md`: Phase 2 completion noted, differences-from-CLI section updated
• `docs/json-schema.md`: new schema + breaking-change section
• `docs/architecture.md`: cmd/prism, formatter, usecase descriptions updated

Deferred to a future PR

• `prompt` and `fetch` commands still go through `internal/usecase` because `pkg/prism.Prompt` returns a plain string and does not yet cover `--format json|markdown` or `--template`. Extending `pkg/prism` with a structured prompt API would unlock the full unification.

Test plan

• `make test` (with `-race`) — all packages pass
• `make lint` — 0 issues
• `make vet` — clean
• Golden test verifies CLI JSON ≡ `pkg/prism.Result` JSON
• New integration tests cover Analyze happy path, IncludePatches, explicit provider, upstream failure, Prompt happy path