Add chain-aware native rotation, compression, and retention for audit/attestation logs

[se-jo-ma]

Context

The append-only audit log is durably fsync'd per write (nautilus/audit/logger.py _flush_sink) but grows without bound. Rotation is delegated entirely to external tooling: docs/how-to/operator-guide.md:174-181 tells operators to "rotate and archive it like any log (e.g. logrotate with copytruncate disabled)". There are no rotation, retention, compression, or size-cap fields anywhere in code (grep for rotat|retention|max_size|maxBytes|RotatingFile|backupCount|gzip over nautilus/audit/ and config/models.py returns nothing).

Unbounded growth is observable in-repo today: audit.jsonl is 1.3MB and nautilus-audit.jsonl is 612KB with no management.

External logrotate is also unsafe for the chained attestation sink. FileSinkSpec (config/models.py:155-170) supports chained: true (hash-linked prev_sha256 + EdDSA JWS via fathom.chained_log.ChainedAttestationLog) and checkpoint_interval, verified offline by nautilus attestation verify (nautilus/cli/attestation.py). A naive external move/truncate breaks the hash chain so verify_chain (used at nautilus/ui/router.py:106) can no longer chain across the cut.

Note: two distinct files are involved.

• The plain audit JSONL — AuditConfig.path (config/models.py:233-236), built as AuditLogger(sink=FileSink(path=audit_path)) at core/broker.py:577. No chain; rotation here is "simple".
• The chained attestation log — FileSinkSpec (config/models.py:155-170). Rotation here MUST be chain-aware.

Task

Add opt-in native rotation (off by default; default behavior unchanged when unconfigured).

• Add a rotation spec (e.g. max_size_mb and/or max_age + compress: gzip + keep: N) to AuditConfig for the plain audit log, and to FileSinkSpec for the attestation log.
• Roll the active file when the size/age threshold is hit; optionally gzip the rotated segment; prune retained segments to keep. Account for Fathom FileSink opening/closing per write — rotation must check the threshold and re-open the active path consistently.
• For chained: true sinks, write a closing checkpoint/anchor record at the rotation boundary so offline nautilus attestation verify can chain across rotated segments.
• Update the operator guide to document the supported rotation modes and the chained-log interaction, replacing the bare logrotate advice.

Where

• nautilus/audit/logger.py (rotation hook around _flush_sink / emit)
• nautilus/core/broker.py (AuditLogger/sink construction, ~line 577; chained-sink wiring ~line 737)
• nautilus/config/models.py (AuditConfig line 233; FileSinkSpec line 155)
• nautilus/cli/attestation.py (ensure verify chains across segments)
• docs/how-to/operator-guide.md:174-181

Acceptance criteria

• With rotation configured, the active audit file rolls at the size/age threshold.
• Rotated segments are optionally gzipped and pruned down to keep.
• For a chained attestation sink, nautilus attestation verify passes across a rotation boundary (closing anchor/checkpoint written at the cut).
• Rotation off (unconfigured) leaves current single-file behavior byte-for-byte unchanged.
• Unit tests cover size/age trigger, gzip, and pruning; an integration test covers chained verification across a rotation boundary.
• Operator guide documents rotation modes and chained-log interaction; bare logrotate advice removed/updated.

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