fix(dispatch): background report signs with --from; coord-send refuses a worktree-derived sender

In a linked worktree basename(git toplevel) is the worktree's name, so the
background child's report was signed `from: dispatch-bg-form` (measured by
the PM on 741ada8) - a mailbox no session holds.

- skills/dispatch: the starter reports by full path with
  `coord-send.sh --from <repo> --to <sender>`; board-selftest §19b gains the
  check (red on 741ada8) plus a known-negative control that runs the same
  predicate against the old "via coord-send" wording.
- coord-send.sh: a DERIVED sender inside a linked worktree exits 2, naming
  --from and the repo the worktree belongs to. Chose refusal over a warning
  because stderr is where a session least looks and the mail would already be
  delivered, and over a silent redirect for the reason the retired
  ktg-plugin-marketplace address is rejected rather than redirected.
  Linked-ness is git's own test (git-dir != git-common-dir), not ".git is a
  file", because a submodule has that too. coord-selftest section 39: 9
  checks, 3 red before the fix; controls for submodule, main checkout,
  explicit --from and a worktree named like its repo.
- Six other scripts derive identity the same way; named as a bounded gap in
  CLAUDE.md, not changed (not ordered).

Suites under /bin/bash 3.2: coord 266, board 497, route 73, orders 116,
guard 54 = 1006 (was 995). npm test 12/12.

Order 20260923T082933Z-466405184-from-.claude.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -26,6 +26,22 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   `--target-pane` stays `yes|no`. board-selftest §19b pins all of it (17
   checks, 14 red before the skill changed).
 
+### Fixed
+
+- **The background starter signs its report with `--from <repo>`, and
+  `coord-send` refuses a sender derived inside a linked git worktree.** In a
+  worktree, `basename(git toplevel)` is the worktree's name, so the child's
+  report was signed `from: dispatch-bg-form` (measured on 741ada8), a mailbox
+  no session holds. The starter now reports by full path with `--from`
+  (board-selftest §19b, red before the skill changed, with a known-negative
+  control). `coord-send` exits 2 on a derived sender inside a linked worktree,
+  naming `--from` and the repo the worktree belongs to. Linked-ness is git's
+  own test (git-dir differs from git-common-dir), so a submodule, the main
+  checkout, an explicit `--from` and a worktree named like its repo all still
+  send (coord-selftest section 39, 9 checks, 3 red before the fix). The six
+  other scripts that derive identity the same way are unchanged and named as a
+  gap in CLAUDE.md.
+
 ## [0.35.0] - 2026-09-18
 
 ### Added