chore(postmortem): remove the retired postmortem skill

Retirement decided 2026-07-14: no consumers — nothing else in the repo
references it, it is not a pipeline phase or dispatch target, and
exactly one report was ever produced.

Removes postmortem/ (SKILL.md, scripts/, tests/). The global symlink
~/.claude/skills/postmortem and a gitignored __pycache__ leftover were
cleaned up on disk; install.sh/uninstall.sh iterate directories
generically and need no change.

Resolution of the issue's open point (archive): docs/postmortems/
turned out to be untracked all along — gitignored by design as per-run
output — so the repo never carried the archive. The single local
report stays on disk untouched, and the .gitignore entry stays (with
an updated comment) so the local archive does not become git-status
noise.

closes #27

.gitignore

@@ -5,5 +5,7 @@
 __pycache__/
 *.pyc
 
-# postmortem skill output — reports are per-run artifacts, not tracked
+# local report archive of the retired postmortem skill (issue #27) —
+# reports were per-run artifacts, never tracked; the dir may still
+# exist locally
 docs/postmortems/