feat(porcelain): implement 9 gap-fill porcelain commands with full test coverage and docs

Adds the following commands, each comprehensively implemented with short+long flags, JSON output, security-hardened inputs, and dedicated test files:

Core VCS gap-fill (Git-equivalent): - muse rebase — content-addressed commit replay (--squash, --abort, --continue, --onto) - muse clean — working-tree cleanup with --dry-run safety guard - muse describe — nearest-tag labelling (BFS walk, --long format) - muse shortlog — commit summary grouped by author/agent (--numbered, --email) - muse bundle — offline commit transfer (create/unbundle/verify/list-heads) - muse content-grep — full-text search across tracked file content (regex, binary skip)

Muse-specific additions: - muse verify — whole-repository integrity (ref/commit/snapshot/object DAG walk + re-hash) - muse snapshot — explicit snapshot management (create/list/show/export as tar.gz/zip) - muse whoami — identity shortcut (thin wrapper over muse auth whoami)

New core modules: - muse/core/describe.py — BFS tag walk, DescribeResult TypedDict - muse/core/verify.py — VerifyFailure/VerifyResult TypedDicts, run_verify() - muse/core/rebase.py — RebaseState TypedDict, state I/O, collect_commits_to_replay(), replay_one()

Tests: 99 new tests across 9 files — unit, integration, end-to-end, stress. Docs: docs/reference/porcelain.md — comprehensive reference with output schemas, flag tables, and composability patterns.

All checks green: mypy (242 files, 0 errors) · typing_audit (0 violations) · pytest (2999 passed).