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>

README.md

@@ -14,7 +14,7 @@ Session A in repo X leaves a message for repo Y; the next session in repo Y gets
 ![Hooks](https://img.shields.io/badge/hooks-1-green)
 ![Skills](https://img.shields.io/badge/skills-4-orange)
 ![CLI scripts](https://img.shields.io/badge/CLI_scripts-8-blue)
-![Selftest checks](https://img.shields.io/badge/selftest_checks-995-blue)
+![Selftest checks](https://img.shields.io/badge/selftest_checks-1006-blue)
 
 ---
 
@@ -202,8 +202,8 @@ Note that raising the inbox's priority (Rule 7) deliberately does **not** widen
 
 ## Development
 
-    bash scripts/coord-selftest.sh              # 257 checks against a throwaway mailbox
-    bash scripts/board-selftest.sh              # 495 checks against a throwaway repo tree
+    bash scripts/coord-selftest.sh              # 266 checks against a throwaway mailbox
+    bash scripts/board-selftest.sh              # 497 checks against a throwaway repo tree
     bash scripts/route-selftest.sh              # 73 checks, incl. the route->board round trip
     bash scripts/orders-selftest.sh             # 116 checks, incl. the 20-way barriered claim race
     bash scripts/state-line-guard-selftest.sh   # 54 checks, incl. the Edit replace_all projection and the ratchet