slash command for daily AtoCore use + backup-restore procedure

Session 2 of the four-session plan. Lands two operational pieces:
the Claude Code slash command that makes AtoCore reachable from
inside any Claude Code session, and the full backup/restore
procedure doc that turns the backup endpoint code into a real
operational drill.

Slash command (.claude/commands/atocore-context.md)
---------------------------------------------------
- Project-level slash command following the standard frontmatter
  format (description + argument-hint)
- Parses the user prompt and an optional trailing project id, with
  case-insensitive matching against the registered project ids
  (atocore, p04-gigabit, p05-interferometer, p06-polisher and
  their aliases)
- Calls POST /context/build on the live AtoCore service, defaulting
  to http://dalidou:8100 (overridable via ATOCORE_API_BASE env var)
- Renders the formatted context pack inline so the user can see
  exactly what AtoCore would feed an LLM, plus a stats banner and a
  per-chunk source list
- Includes graceful failure handling for network errors, 4xx, 5xx,
  and the empty-result case
- Defines a future capture path that POSTs to /interactions for the
  Phase 9 reflection loop. The current command leaves capture as
  manual / opt-in pending a clean post-turn hook design

.gitignore changes
------------------
- Replaced wholesale .claude/ ignore with .claude/* + exceptions
  for .claude/commands/ so project slash commands can be tracked
- Other .claude/* paths (worktrees, settings, local state) remain
  ignored

Backup-restore procedure (docs/backup-restore-procedure.md)
-----------------------------------------------------------
- Defines what gets backed up (SQLite + registry always, Chroma
  optional under ingestion lock) and what doesn't (sources, code,
  logs, cache, tmp)
- Documents the snapshot directory layout and the timestamp format
- Three trigger paths in priority order:
  - via POST /admin/backup with {include_chroma: true|false}
  - via the standalone src/atocore/ops/backup.py module
  - via cold filesystem copy with brief downtime as last resort
- Listing and validation procedure with the /admin/backup and
  /admin/backup/{stamp}/validate endpoints
- Full step-by-step restore procedure with mandatory pre-flight
  safety snapshot, ownership/permission requirements, and the
  post-restore verification checks
- Rollback path using the pre-restore safety copy
- Retention policy (last 7 daily / 4 weekly / 6 monthly) and
  explicit acknowledgment that the cleanup job is not yet
  implemented
- Drill schedule: quarterly full restore drill, post-migration
  drill, post-incident validation
- Common failure mode table with diagnoses
- Quickstart cheat sheet at the end for daily reference
- Open follow-ups: cleanup script, off-Dalidou target,
  encryption, automatic post-backup validation, incremental
  Chroma snapshots

The procedure has not yet been exercised against the live Dalidou
instance — that is the next step the user runs themselves once
the slash command is in place.

.gitignore

@@ -10,4 +10,6 @@ htmlcov/
 .coverage
 venv/
 .venv/
-.claude/
+.claude/*
+!.claude/commands/
+!.claude/commands/**