fix(issue-13): refuse to purge the conversation the caller is running in

Review of 0282312 (planner-reviewer-claude-01, N1): conversation_in_use
excluded the purge's whole ancestor lineage, and that lineage includes the
agent CLI that launched the skill. An orchestrator purging its own UUID
from its own session was therefore never refused - the only process naming
the UUID was excluded, and claude keeps no transcript open for lsof - so
its live conversation was deleted, contrary to what orc-onboard SKILL.md
promises.

- base.conversation_in_use: in use when one of the caller's session-id
  variables (CLAUDE_CODE_SESSION_ID, ANTIGRAVITY_CONVERSATION_ID,
  HERMES_SESSION_ID, GROK_SESSION_ID, OPENCODE_SESSION_ID) equals the uuid;
  the command-line check now excludes only the purge's own helper chain and
  stops at the nearest agent CLI ancestor (basename claude/agy/hermes/grok/
  opencode/codex, read like detect_nearest_agent), which counts as a holder
  when it names the uuid; the lsof check is unchanged
- tests: the non-agent helper chain is excluded but an agent ancestor is
  counted; the purge CLI is refused under an agent CLI naming the uuid and
  with each of the five session variables set to it; orc_onboard --remove
  --purge-conversation --yes is refused from the orchestrator's own session
  (exit 5, registration kept). The eight new cases fail on 0282312
- job_subscriber --registry-dir now defaults to default_registry_dir() like
  the other entry points (review R-d)
- orc-onboard SKILL.md spells out how the calling session is recognised;
  VERSIONS.md v4.4.3 item 2 and the new-test count (24)

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HCGpJXhSZasJLU38ksDghe

.agents/skills/multi-agent-mux-orc-onboard/SKILL.md

@@ -26,7 +26,7 @@ Registers an orchestrator session UUID into the `orchestrator_uuids` list of `.m
 ### Options
 - `--uuid <uuid>`: Explicitly register the specified orchestrator session UUID.
 - `--remove <uuid>`: Remove the specified UUID from the `orchestrator_uuids` list.
-- `--purge-conversation`: With `--remove`, also delete the conversation's on-disk artifacts through its agent adapter (Issue #13). Irreversible — requires `--yes`; refused (exit `5`, registration untouched) while a live process still references the conversation, e.g. when run from inside that very session.
+- `--purge-conversation`: With `--remove`, also delete the conversation's on-disk artifacts through its agent adapter (Issue #13). Irreversible — requires `--yes`; refused (exit `5`, registration untouched) while a live process still references the conversation — including the session the command itself runs in: its session-id variable (`CLAUDE_CODE_SESSION_ID`, `ANTIGRAVITY_CONVERSATION_ID`, `HERMES_SESSION_ID`, `GROK_SESSION_ID`, `OPENCODE_SESSION_ID`) equals the UUID, or the nearest agent CLI ancestor names it on its command line. An orchestrator therefore cannot purge its own conversation from inside that very session.
 - `--yes`: Skip the `--purge-conversation` confirmation (without it the script prints the danger notice and exits `4`).
 - `--agent <name>`: Adapter to purge with (`claude|agy|hermes|grok|opencode|codex`). Default `auto`: each adapter is asked whether it owns an artifact for the UUID (`agy` → `~/.gemini/antigravity-cli/conversations/<uuid>.db` etc.). Needed because `orchestrator_uuids` stores bare UUIDs, not the agent kind.
 - `--list`: Display all currently registered orchestrator UUIDs.