coord: add msg purge --before command for cleaning dealt-with messages

Adds c_msg_purge to coord.py + SKILL.md doc. Deletes coordination messages older than a
date cutoff via DELETE /api/coord/messages/{id}. Safety: --before is required (can't wipe
the store by accident), DRY-RUN by default (previews; --yes to actually delete), optional
--to scopes to one recipient session, paginates over the API's 1000-row limit cap, logs
partial failures. Replaces the ad-hoc curl loop used to purge 208 stale messages this session.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.claude/skills/coord/SKILL.md

@@ -27,6 +27,7 @@ bash "$CLAUDETOOLS_ROOT/.claude/scripts/py.sh" "$CLAUDETOOLS_ROOT/.claude/skills
 | `msg send <to> "<subject>" --body-file <path>` | Same, body from a file (use for long/multi-line bodies — avoids shell-quoting breakage). |
 | `msg inbox [--all]` | Your unread messages (or `--all`). Print these prominently per the coord rule. |
 | `msg read <id>` | Mark a message read. |
+| `msg purge --before YYYY-MM-DD [--to <session>] [--yes]` | Delete dealt-with messages older than the cutoff date. **DRY-RUN by default** (previews what would go); add `--yes` to actually delete. `--to` scopes to one recipient session. Refuses to run without `--before` (can't wipe the store by accident). Destructive + fleet-wide — coordination msgs are ephemeral; the durable record is in session logs. |
 | `todo add "<text>" [--user U] [--project KEY] [--parent ID] [--source TXT] [--auto]` | Create a todo (auto-fills `created_by_user`/`created_by_machine`). `--text-file` for long text. |
 | `todo list [--user U] [--project KEY] [--status pending\|done\|all]` | List todos. |
 | `todo done <id>` | Mark a todo done (sets `completed_by`). |