[Feature] Add ccpr_review MCP tool

[hidetzu]

Summary

Expose ccpr review --format json as an MCP tool named ccpr_review, served by the existing ccpr-mcp binary. The tool returns PR metadata, comments, and the
local-Git-generated diff in a single structured response.

Motivation

• This is the headline use case in CLAUDE.md — "developer runs ccpr review --format json, passes the output to Claude Code, and Claude generates review comments."
  Today this still requires a manual pipe.
• ccpr_list is already wired up; extending the same internal/app pattern to review is the natural next step and validates that the shared use case approach scales.
• review --format json is already an object-typed payload ({metadata, comments, diff}), so it maps directly onto an MCP tool output schema without a wrapper.

Proposed Solution

Scope

• New MCP tool: ccpr_review.
• Input: PR URL preferred (mirroring CLAUDE.md "The primary input is a CodeCommit PR URL"). Also accept repo + prId + region as secondary, matching the CLI's --repo /
  --pr-id flags.
• Output: the existing ccpr review --format json schema — top-level object with metadata, comments, diff. No wrapper needed.
• Reuse the internal use case: extract the CLI's review pipeline into internal/app/review.go so the CLI and MCP paths share validation, AWS resolution, comment fetching,
  and diff generation.
• cmd/ccpr-mcp/main.go registers ccpr_review alongside ccpr_list.

Out of scope

• --format patch and --format summary over MCP. MCP consumers want structured data; the CLI keeps these formats for human use.
• Line-level comment objects beyond what review --format json already returns.
• Any change to the ccpr CLI behavior or its existing JSON output.

Implementation plan

1. Update docs first, per the spec-driven rule in CLAUDE.md:
   - docs/use-cases.md: scenario where Claude Code fetches a full PR review payload via MCP.
   - docs/requirements.md: extend FR-18 (or add a sibling) to cover ccpr_review.
   - docs/spec.md: input schema (URL vs. repo/prId/region), output schema (reuse review --format json), error handling.
2. Extract the review use case into internal/app/review.go and migrate cmd/ccpr/review.go to call it.
3. Register ccpr_review in cmd/ccpr-mcp/main.go.
4. Extend the existing newServer() smoke test so a tool-schema regression in ccpr_review is caught at build time, just like ccpr_list.
5. Update README.md / README.ja.md with the new MCP tool.

Acceptance criteria

• ccpr_review is callable from an MCP client and returns the same payload as ccpr review --format json.
• The CLI ccpr review continues to work with no behavioral or output changes (existing golden tests in cmd/ccpr/testdata/ still pass).
• ccpr_list is unaffected.
• Docs (use-cases / requirements / spec / json-schema / README) updated.
• make build / make build-mcp / make test / make vet / make lint all pass.