feat(mcp): codegraph_review_context — structured PR-review context from a diff

Adds a new MCP tool that takes a unified diff and returns structured
review context for an LLM-driven PR reviewer. Codegraph becomes a
substrate for Greptile/CodeRabbit-style products without itself doing
the synthesis.

## What it returns

Per changed file:
  - status (added / modified / deleted / renamed)
  - hunks (line ranges)
  - affected symbols (line-range overlap with hunks)
  - tests covering the file (via tests-edges; graceful no-op if absent)

Per affected symbol:
  - signature, docstring
  - top-N callers, top-N callees
  - impact-radius node count

## Components

- src/review/diff-parser.ts: pure unified-diff parser
- src/review/index.ts: buildReviewContext + co-change warnings
- src/index.ts: CodeGraph.buildReviewContext public API
- src/mcp/tools/review-context.ts: ToolModule (post-colbymchenry#117 form)
- src/mcp/tools.ts: handleReviewContext + serializeReviewContextWithinCap
  (progressive-trim JSON serializer that keeps output <= MAX_OUTPUT_LENGTH
  while preserving JSON parseability)
- __tests__/review-context.test.ts: 25 tests

Output is JSON; the LLM consumer does the synthesis.

This is the post-colbymchenry#117 (per-tool MCP registry) form — the original
PR's case-switch dispatch is replaced by a ToolModule entry plus
a 'handleReviewContext' HandlerKey.

Tests that exercise co-change warnings include a graceful inline
ALTER TABLE for commit_count, since that column lands with colbymchenry#112's
centrality+churn migration. After colbymchenry#112 lands the ALTER becomes a
no-op (column already exists).

Author: andreinknv

__tests__/mcp-tool-registry.test.ts

@@ -37,7 +37,7 @@ describe('MCP tool registry — single source of truth', () => {
     expect(fromExport).toEqual(fromRegistry);
   });
 
-  it('all 9 main-line tools are registered (regression guard)', () => {
+  it('all 10 main-line tools are registered (regression guard)', () => {
     const expected = [
       'codegraph_callees',
       'codegraph_callers',
@@ -46,6 +46,7 @@ describe('MCP tool registry — single source of truth', () => {
       'codegraph_files',
       'codegraph_impact',
       'codegraph_node',
+      'codegraph_review_context',
       'codegraph_search',
       'codegraph_status',
     ];