feat(code): supercharge code plugin with 9 new semantic commands

Add a full suite of code-domain CLI commands that are structurally impossible in Git — treating the codebase as a typed, content-addressed symbol graph rather than a bag of text lines.

New shared helper: - muse/plugins/code/_query.py: symbols_for_snapshot(), walk_commits(), walk_commits_range(), flat_symbol_ops(), touched_files(), file_pairs()

New analysis commands (read-only): - muse grep: search the symbol graph by name, kind, language — not text - muse blame: per-symbol attribution — one answer per function, not per line - muse hotspots: symbol churn leaderboard — which functions change most - muse stable: symbol stability leaderboard — the bedrock of your codebase - muse coupling: semantic file co-change analysis — hidden dependencies - muse compare: deep semantic diff between any two historical snapshots - muse languages: language + symbol-type breakdown of any snapshot

New agent-scale commands: - muse patch: surgical per-symbol modification — modify exactly one named symbol - muse query: symbol graph predicate DSL (kind/language/name/file/hash)

All commands support --json for pipeline integration. All pass mypy strict, typing_audit --max-any 0, and 797 pytest tests.

Update docs/demo/tour-de-force-code.md with all 12 commands as a full paradigm-shift narrative. Update docs/demo/README.md to reflect the complete command matrix.