fix(skill): stale user-level skill copy shadowed injections; seed the preamble

Two live failures from one root cause: Claude Code loads a same-named
user-level skill (~/.claude/skills/codeman, written once by `codeman skill
install`) over the fresh per-case copy, and nothing ever refreshed it. A
stale Aug-9 copy (pre fast-path, pre lineage header) made every agent-driven
spawn run the old recipes: workers spawned serially with pid polls and
without X-Codeman-Parent-Session, so the web UI drew no lineage arcs.

- refreshUserAgentSkill(): session create now refreshes a marker-owned
  user-level copy (refresh-only: absent copies are not installed,
  foreign/symlink copies stay untouched).
- seedAgentSessionPreamble(): local claude session create pre-seeds the
  skill's preamble into ${XDG_CACHE_HOME:-~/.cache}/codeman-agent-<id>.sh,
  single-sourced from the new skills/codeman/preamble.sh, so the skill's §0
  bootstrap collapses to a two-line loader instead of a ~150-line paste the
  model has to type out (measured ~47s of generation per run).
- SKILL.md: §0 now leads with the loader and keeps the full block as the
  stale/missing fallback; explicit verbatim-paste warning (a hand-assembled
  preamble is how the header and the fast-path functions got lost);
  spawn_worker also sends parentSessionId in the body as defense in depth;
  preamble stamp bumped to 1.18.3 so pre-fix cached preambles self-heal.
- test/agent-skill.test.ts pins preamble.sh byte-identical to the SKILL.md
  heredoc and covers seeding (XDG + HOME fallback, 0600) and the user-level
  refresh (absent/stale/foreign).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

skills/codeman/reference/recipes.md

@@ -21,7 +21,7 @@ by sourcing the preamble file the §0 bootstrap wrote, and checking its version
 
 ```bash
 . "${XDG_CACHE_HOME:-$HOME/.cache}/codeman-agent-$CODEMAN_SESSION_ID.sh" 2>/dev/null
-[ "${CODEMAN_PREAMBLE:-}" = 1.18.2 ] || { echo "preamble missing or stale; re-run the §0 bootstrap"; exit 1; }
+[ "${CODEMAN_PREAMBLE:-}" = 1.18.3 ] || { echo "preamble missing or stale; re-run the §0 bootstrap"; exit 1; }
 ```
 
 Do **not** re-paste the preamble body into each call. Sourcing it is what retires the