feat(hook): let a non-git surface declare its identity via CLAUDE_COORD_REPO

0.6.0 removed the working-directory fallback, which was right, but it left
every non-git working surface with nothing to derive from. The read path
declines silently by design - the hook must never fail a session - so such a
surface simply stops seeing its inbox: no error, no exit code, nothing. That is
the same loss-looks-like-normal shape 0.6.0 set out to remove, reintroduced one
layer up.

The hook now forwards CLAUDE_COORD_REPO verbatim as --repo. This is not the
fallback returning, and the distinction is the whole point: the fallback GUESSED
a name from wherever the session happened to stand, while a declaration is
written down in that directory's settings, readable back, and deletable.
Identity stays a choice someone made.

Forwarding as --repo rather than reimplementing anything means it inherits every
engine rule, including that an explicit override never claims <repo>/.origin -
otherwise a surface borrowing a name would steal the claim from the checkout
that owns it. Both halves are pinned by tests, and the shared test helper now
deletes CLAUDE_COORD_REPO unless a test asks for it, so a value set globally
later cannot silently satisfy the tests that prove the hook resolves nothing on
its own.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01U6EixQo6hpoRCVtiAXdnFs

package.json

@@ -1,6 +1,6 @@
 {
   "name": "repo-mailbox",
-  "version": "0.6.0",
+  "version": "0.7.0",
   "private": true,
   "type": "module",
   "engines": {