docs(postmortem): note the worktree slug caveat for the default log dir

The default project log dir is derived from cwd, so a session run inside a
git worktree resolves to a different slug than the primary clone. Document
that, and point at --project-dir / --file to grade a worktree run.

closes #18

postmortem/SKILL.md

@@ -80,6 +80,14 @@ dir — i.e. the session you are in (it will be flagged `active`,
 meaning totals are partial). To grade a finished run, pass its
 `--session <uuid>`.
 
+The default log dir is derived from cwd (the path with `/` replaced
+by `-`), so a session run inside a git worktree resolves to a
+*different* slug than the primary clone, and after exiting a worktree
+the default again follows the current cwd. To grade a worktree run
+from the primary clone, pass `--project-dir
+~/.claude/projects/<worktree-slug>` (or `--file <path>` to bypass slug
+derivation entirely).
+
 The script emits one JSON object on stdout. That JSON — not the
 raw log — is your evidence base.