feat(tests+docs): supercharge plumbing test suite and update reference doc

Tests — 11 new test files covering the 11 previously untested plumbing commands, plus cross-command integration tests and a stress suite:

test_plumbing_hash_object.py — SHA-256 correctness, --write, 2 MiB file test_plumbing_cat_object.py — raw streaming, info format, large blobs test_plumbing_rev_parse.py — HEAD/branch/prefix resolution, errors test_plumbing_update_ref.py — update, create, delete, --no-verify test_plumbing_ls_files.py — HEAD/--commit, text format, 500-file manifest test_plumbing_commit_tree.py — create, parents, merge commit, metadata test_plumbing_read_commit.py — full ID, prefix, schema validation, 100-batch test_plumbing_read_snapshot.py — retrieval, manifest, 1000-file stress test_plumbing_commit_graph.py — BFS, --stop-at, --max, text format, 200-chain test_plumbing_pack_unpack.py — schema, --have pruning, round-trip, 50-obj stress test_plumbing_integration.py — 8 cross-command pipelines (hash→cat→verify, commit-tree→update-ref→rev-parse, pack→unpack transport, snapshot-diff↔ls-files, show-ref↔for-each-ref, symbolic-ref→rev-parse→ read-commit, merge-base→snapshot-diff, commit-graph→name-rev) test_plumbing_stress.py — 500-commit BFS, 300-deep merge-base, 200-commit name-rev, 2000-file snapshot-diff, 200-object verify, 100-branch for-each-ref/show-ref, 100-commit/100-object pack round-trip, 200-commit sequential read-commit

Total: 2900 tests (was 2743) — 157 new tests, all green.

Docs — docs/reference/plumbing.md: - Add Quick Index table (all 24 commands, one-line description, anchor link) - Standardise headings: every command now uses ### `name` — slug style - Fix bug: pack-objects output showed data_b64 (wrong); corrected to content_b64 - Merge duplicate commit-graph sections into one complete entry with all 7 flags - Add --count / --first-parent / --ancestry-path to commit-graph flag table - Fix name-rev docs: distance=0 emits bare branch name, not <branch>~0 - Add "Composability Patterns — Advanced" section with 6 new pipeline recipes for symbolic-ref, for-each-ref, check-ref-format, verify-pack, name-rev, snapshot-diff, and the stale-branch audit pattern

Verification: mypy 0 errors, typing_audit 0 violations, pytest 2900/2900.