feat: read Claude Code sessions directly from the Claude home directory

[scode]

Second step of the hookless transition (lore/hookless-migration.md): distill no longer depends on the SessionEnd
hook having copied a transcript — it reads Claude Code's own session store, with the Codex watermark model generalized
to both harnesses. The hook keeps running in this revision; a session present in both places is emitted once.

Two semantics deserve attention beyond the diff:

• last_distilled doubles as a floor for the scan, so the first scan-enabled distill does not replay the retention
  window the legacy flow already learned from. For the same reason fresh installs now stamp install time instead of
  epoch 0, which would ingest weeks of pre-install history the old system never saw.
• mark-distilled now adopts the scan-start time distill staged rather than its own wall clock. That closes a window
  where a session born between the scan and the mark would sit below the floor forever — the legacy flow silently
  deleted such logs as obsolete, so this fixes an inherited loss, not just a new one.

The scanner treats projects/ as an undocumented, untrusted layout: UUID-stem allowlist, symlinks rejected at read
time via handle identity (not just at discovery), and labels sanitized before they reach tag-attribute position.

changelog: include