feat(engine): harden mailbox CLIs for v0.2.0

- Atomic delivery: create the temp file inside the destination dir
  (dot-prefixed, invisible to the inbox glob) so the final rename never
  crosses filesystems and readers never see a half-written message.
- Reject . and .. explicitly in the --reply-to and coord-done name
  guards instead of relying on downstream failure.
- Add -h/--help to coord-inbox.sh (uniform across the three CLIs).
- Close selftest gaps: default mailbox path via HOME fallback, malformed
  frontmatter on the read path, read-only destination dir. 48 -> 64
  checks, all green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018fduZz8otpU3W3rhfoPD6t

scripts/coord-inbox.sh

@@ -23,6 +23,7 @@ while [ $# -gt 0 ]; do
     # bash 3.2: `shift 2` past the end of $# is a no-op -> would loop forever.
     --repo) [ $# -ge 2 ] || { echo "coord-inbox: --repo requires a value" >&2; exit 2; }
             REPO="$2"; shift 2 ;;
+    -h|--help) grep '^#' "$0" | sed 's/^# \{0,1\}//'; exit 0 ;;
     *) shift ;;
   esac
 done