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

hooks/scripts/session-start.mjs

@@ -0,0 +1,42 @@
+#!/usr/bin/env node
+// coord - SessionStart hook: inject this repo's pending coordination inbox
+// (directed messages + unseen broadcasts) as additionalContext.
+//
+// Thin Node wrapper (marketplace convention: hooks are .mjs) around the bash
+// engine scripts/coord-inbox.sh, which owns the mailbox semantics and is
+// covered by scripts/coord-selftest.sh. Zero dependencies. Always exits 0 -
+// a broken mailbox must never block a session.
+
+import { execFileSync } from 'node:child_process';
+import { basename, dirname, join } from 'node:path';
+import { fileURLToPath } from 'node:url';
+
+function emit(context) {
+  const out = { continue: true };
+  if (context) {
+    out.hookSpecificOutput = { hookEventName: 'SessionStart', additionalContext: context };
+  }
+  process.stdout.write(JSON.stringify(out) + '\n');
+}
+
+try {
+  const pluginRoot = process.env.CLAUDE_PLUGIN_ROOT
+    || join(dirname(fileURLToPath(import.meta.url)), '..', '..');
+
+  let repoRoot = '';
+  try {
+    repoRoot = execFileSync('git', ['rev-parse', '--show-toplevel'],
+      { stdio: ['ignore', 'pipe', 'ignore'], encoding: 'utf8' }).trim();
+  } catch {
+    // not a git repo: fall back to the working directory below
+  }
+  if (!repoRoot) repoRoot = process.cwd();
+
+  const inbox = execFileSync('bash',
+    [join(pluginRoot, 'scripts', 'coord-inbox.sh'), '--repo', basename(repoRoot)],
+    { stdio: ['ignore', 'pipe', 'ignore'], encoding: 'utf8' });
+
+  emit(inbox.trim() ? '== Repo coordination (unread messages) ==\n' + inbox : '');
+} catch {
+  emit('');
+}