Add versioned pack identity/provenance metadata and surface it via loader, CLI, and audit records

[se-jo-ma]

Context

For compliance attestations (HIPAA/NIST/CMMC), knowing which version of a rule pack produced a decision is mandatory metadata — auditors will ask. Today the four compliance packs are anonymous directories: RulePackLoader.discover/load (src/fathom/packs.py, 67 lines) only resolves a directory path from the fathom.packs entry point and loads the templates/ modules/ functions/ rules/ subdirs. No pack-level version, source citation, or maintainer is read anywhere, and AuditRecord.metadata (src/fathom/models.py:364) carries no pack identity.

Strong existing precedent (reuse it, don't reinvent): The SSVC pack already implements exactly this pattern via SSVC_META in src/fathom/rule_packs/ssvc/__init__.py — a dict with version, source, source_sha256, branches_source — exposed as a module attribute, asserted as an ssvc_meta fact that survives an evaluate() round-trip (tests/rule_packs/test_ssvc.py), with SHA-pinned provenance and a documented version-bump discipline. This issue is about generalizing that pattern to the other four shipped packs and surfacing it through the loader, CLI, and audit path. Prefer a Python __manifest__/PACK_META module attribute (consistent with SSVC) over inventing a separate pack.yaml.

Note: rule YAML files already carry a top-level version: "1.0" (read into RulesetDefinition.version), but it is not surfaced at the pack level.

This is distinct from the signed-bundle distribution work in "[P3] Hot-reload + signed bundle distribution + impact analysis" (issue #82): this is in-tree pack identity/provenance, not artifact signing/distribution.

Task

• Define a small validated pack-metadata model (Pydantic) with name, version (semver), description, controls/sources, maintainer, modeled on the existing SSVC_META shape.
• Have RulePackLoader read and validate the metadata when loading a pack and expose it on the loaded engine (or return it from the loader).
• Backfill metadata for the four shipped packs (owasp_agentic, nist_800_53, hipaa, cmmc), matching the SSVC convention. Note owasp_agentic/__init__.py is currently docstring-only (no get_* functions), unlike the other three — bring it in line.
• Surface installed pack names + versions via the CLI. The existing info command takes a directory path and lists constructs (src/fathom/cli.py:303), so a new packs command that enumerates the fathom.packs entry points + versions is cleaner than overloading info.
• Include pack name+version in AuditRecord.metadata so attestations cite the exact pack release.

Where

• src/fathom/packs.py — metadata discovery/validation in RulePackLoader.
• src/fathom/rule_packs/ssvc/__init__.py — reference pattern (SSVC_META); do not regress.
• src/fathom/rule_packs/{owasp_agentic,nist_800_53,hipaa,cmmc}/__init__.py — backfill metadata.
• src/fathom/cli.py — new packs command (and/or info augmentation).
• src/fathom/models.py — metadata model; ensure pack id flows into AuditRecord.metadata.
• tests/test_packs.py (new) — loader/validation tests; the per-pack tests live in tests/test_{owasp,nist,hipaa,cmmc}_pack.py and tests/rule_packs/test_ssvc.py.

Acceptance criteria

• Loading a pack exposes its validated version/metadata.
• The four shipped packs have valid metadata consistent with the SSVC convention; owasp_agentic no longer diverges from the other packs' module shape.
• A new fathom packs command lists installed pack names + versions.
• An evaluation's AuditRecord.metadata cites the pack name+version.
• uv run pytest, uv run ruff check src/ tests/, and uv run mypy src/ pass.

Size: M