fix: chroma restore bind-mount bug + consolidate docs

Two fixes from the 2026-04-09 first real restore drill on Dalidou,
plus the long-overdue doc consolidation I should have done when I
added the drill runbook instead of creating a duplicate.

## Chroma restore bind-mount bug (drill finding)

src/atocore/ops/backup.py: restore_runtime_backup() used to call
shutil.rmtree(dst_chroma) before copying the snapshot back. In the
Dockerized Dalidou deployment the chroma dir is a bind-mounted
volume — you can't unlink a mount point, rmtree raises
  OSError [Errno 16] Device or resource busy
and the restore silently fails to touch Chroma. This bit the first
real drill; the operator worked around it with --no-chroma plus a
manual cp -a.

Fix: clear the destination's CONTENTS (iterdir + rmtree/unlink per
child) and use copytree(dirs_exist_ok=True) so the mount point
itself is never touched. Equivalent semantics, bind-mount-safe.

Regression test:
tests/test_backup.py::test_restore_chroma_does_not_unlink_destination_directory
captures Path.stat().st_ino of the dest dir before and after
restore and asserts they match. That's the same invariant a
bind-mounted chroma dir enforces — if the inode changed, the
mount would have failed. 11/11 backup tests now pass.

## Doc consolidation

docs/backup-restore-drill.md existed as a duplicate of the
authoritative docs/backup-restore-procedure.md. When I added the
drill runbook in commit 3362080 I wrote it from scratch instead of
updating the existing procedure — bad doc hygiene on a project
that's literally about being a context engine.

- Deleted docs/backup-restore-drill.md
- Folded its contents into docs/backup-restore-procedure.md:
  - Replaced the manual sudo cp restore sequence with the new
    `python -m atocore.ops.backup restore <STAMP>
    --confirm-service-stopped` CLI
  - Added the one-shot docker compose run pattern for running
    restore inside a container that reuses the live volume mounts
  - Documented the --no-pre-snapshot / --no-chroma / --chroma flags
  - New "Chroma restore and bind-mounted volumes" subsection
    explaining the bug and the regression test that protects the fix
  - New "Restore drill" subsection with three levels (unit tests,
    module round-trip, live Dalidou drill) and the cadence list
  - Failure-mode table gained four entries: restored_integrity_ok,
    Device-or-resource-busy, drill marker still present,
    chroma_snapshot_missing
  - "Open follow-ups" struck the restore_runtime_backup item (done)
    and added a "Done (historical)" note referencing 2026-04-09
  - Quickstart cheat sheet now has a full drill one-liner using
    memory_type=episodic (the 2026-04-09 drill found the runbook's
    memory_type=note was invalid — the valid set is identity,
    preference, project, episodic, knowledge, adaptation)

## Status doc sync

Long overdue — I've been landing code without updating the
project's narrative state docs.

docs/current-state.md:
- "Reliability Baseline" now reflects: restore_runtime_backup is
  real with CLI, pre-restore safety snapshot, WAL cleanup,
  integrity check; live drill on 2026-04-09 surfaced and fixed
  Chroma bind-mount bug; deploy provenance via /health build_sha;
  deploy.sh self-update re-exec guard
- "Immediate Next Focus" reshuffled: drill re-run (priority 1) and
  auto-capture (priority 2) are now ahead of retrieval quality work,
  reflecting the updated unblock sequence

docs/next-steps.md:
- New item 1: re-run the drill with chroma working end-to-end
- New item 2: auto-capture conservative mode (Stop hook)
- Old item 7 rewritten as item 9 listing what's DONE
  (create/list/validate/restore, admin/backup endpoint with
  include_chroma, /health provenance, self-update guard,
  procedure doc with failure modes) and what's still pending
  (retention cleanup, off-Dalidou target, auto-validation)

## Test count

226 passing (was 225 + 1 new inode-stability regression test).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/atocore/ops/backup.py

@@ -337,9 +337,22 @@ def restore_runtime_backup(
         src_chroma = Path(chroma_snapshot_path)
         if src_chroma.exists() and src_chroma.is_dir():
             dst_chroma = _config.settings.chroma_path
-            if dst_chroma.exists():
-                shutil.rmtree(dst_chroma)
-            shutil.copytree(src_chroma, dst_chroma)
+            # Do NOT rmtree the destination itself: in a Dockerized
+            # deployment the chroma dir is a bind-mounted volume, and
+            # unlinking a mount point raises
+            #   OSError [Errno 16] Device or resource busy.
+            # Instead, clear the directory's CONTENTS and copytree into
+            # it with dirs_exist_ok=True. This is equivalent to an
+            # rmtree+copytree for restore purposes but stays inside the
+            # mount boundary. Discovered during the first real restore
+            # drill on Dalidou (2026-04-09).
+            dst_chroma.mkdir(parents=True, exist_ok=True)
+            for item in dst_chroma.iterdir():
+                if item.is_dir() and not item.is_symlink():
+                    shutil.rmtree(item)
+                else:
+                    item.unlink()
+            shutil.copytree(src_chroma, dst_chroma, dirs_exist_ok=True)
             chroma_restored = True
 
     restored_integrity_ok = False