feat(cli): add stats command for indexed codebase

[r0h1tb]

Closes #13

Adds ast-rag stats, reporting the four things the issue asks for — node counts by kind, edge counts by type, language distribution, file count — plus --json for scripting.

$ ast-rag stats
─────────────────── AST-RAG Index Statistics ───────────────────
  Files indexed: 103

Nodes (921 total)
  Field                     414
  Method                    368
  Class                     100
  Constructor                27
  Interface                   9
  Enum                        3

Edges (951 total)
  CONTAINS_FIELD            409
  CONTAINS_METHOD           392
  CALLS                     150

Languages
  java                      921  (100%)

That output is from a real 103-file Spring Boot codebase, not a fixture.

Two details worth flagging

Edges are grouped by the kind property, not by relationship type. batch_upsert_edges writes every edge as a generic :EDGE and carries the semantic type as a property, so type(r) would collapse everything into a single EDGE bucket and the command would look broken. The query uses coalesce(r.kind, type(r)) so it still degrades sensibly if typed relationships are ever introduced. There's a test pinning this specifically, so it can't quietly regress.

CurrentVersion is excluded from node counts. It's the graph's bookkeeping pointer rather than a code node — including it made the total read 922 for a codebase with 921 nodes.

Tests

5 new tests in tests/test_stats_command.py, all against a mock driver so they need no live Neo4j: totals and shaping, the two query-shape guards above, the empty-index hint, and --json emitting parseable JSON.

Verification

ruff check ast_rag/ tests/     All checks passed!
ruff format --check            76 files already formatted
pytest tests/                  3 failed, 179 passed, 2 xfailed

Baseline on main is 3 failed, 174 passed — same three pre-existing failures (fixed separately in #51), plus the 5 new tests.

Independent of #50, #51 and #52; branches from main.

[r0h1tb]

CI on this PR is red at the Lint with ruff step, which is pre-existing and unrelated to these changes — main fails identically.

Ruff is unpinned and [tool.ruff] never set select, so a newer ruff widened its default rule set and the tree went from clean to 496 findings with no code change. Running today's ruff against 16ae8bd — the last commit whose CI run was green — also reports 496.

Because lint runs before pytest, the Run tests step is skipped, so CI hasn't actually executed the suite since 2026-07-27.

#51 fixes that and is passing. Once it lands, this PR should go green on a re-run; I've verified all three CI steps locally in the meantime.

[lexasub]

Good pr