feat(plugin): package as a Claude Code marketplace plugin

- .claude-plugin/plugin.json v0.1.0 (auto_discover, MIT)
- SessionStart hook: thin zero-dep .mjs wrapper (marketplace convention)
  around scripts/coord-inbox.sh, emitting the additionalContext envelope;
  always exits 0. Smoke-tested: empty mailbox -> bare continue, pending
  message -> injected with UNTRUSTED framing.
- coord-send skill bundled; examples and description use generic repo
  names only (coordination metadata never reaches a public surface).
- README (English; documents the deliver-until-done lifecycle correctly,
  review §7), CHANGELOG, LICENSE (MIT), CLAUDE.md, package.json +
  node --test wrapper around the bash selftest.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HBbjgS5A55RVavoyjJC4FX

tests/selftest.test.mjs

@@ -0,0 +1,13 @@
+// Node wrapper (marketplace convention: node --test) around the bash
+// selftest, which owns every mailbox assertion. The selftest runs against a
+// throwaway mailbox (mktemp) and exits non-zero on any failing check.
+import { test } from 'node:test';
+import { execFileSync } from 'node:child_process';
+import { dirname, join } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+const root = join(dirname(fileURLToPath(import.meta.url)), '..');
+
+test('coord bash selftest passes', () => {
+  execFileSync('bash', [join(root, 'scripts', 'coord-selftest.sh')], { encoding: 'utf8' });
+});