feat(chatarchive): add incremental archive with file modification time tracking

[CodeMonkeyCybersecurity]

Problem

Currently, every archive run re-hashes ALL discovered files to check against the manifest. For large source directories (e.g. ~/Dev with thousands of files), this is O(n) SHA-256 computations on every run, even when nothing has changed.

Proposed fix

Track file modification time (mtime) and size in the manifest entries. On subsequent runs:

1. If mtime+size match the manifest entry → skip hashing entirely
2. If mtime or size differ → re-hash and check for content changes
3. New files → hash as usual

This is the same approach git uses for its index (stat cache).

References

• Git index format: mtime+size as stat cache
• Go os.FileInfo.ModTime() and os.FileInfo.Size()

Acceptance criteria

• Manifest entries include mod_time and size_bytes fields
• Subsequent runs skip hashing when mtime+size match
• Performance benchmark: measure hashing savings on 1000-file directory
• Backward compatible: old manifests without mtime fall back to full hash

[CodeMonkeyCybersecurity]

SOAPIER session note (2026-03-20)

Objective assessment

• Reviewed pkg/chatarchive/archive.go and pkg/chatarchive/manifest.go. Current runs hash every discovered file before comparing against the manifest, so unchanged trees still pay full SHA-256 cost.

Analysis

• This is a valid performance follow-up. It should stay coordinated with manifest evolution work so stat-cache fields remain backward compatible with existing manifests.

Related

• Sibling follow-ups: Add schema versioning and migration support for chat archive manifests #88, feat(chatarchive): add --format flag for machine-readable output (JSON) #92, chore(chatarchive): add golangci-lint configuration for chatarchive package #94.