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

CLAUDE.md

@@ -0,0 +1,59 @@
+# coord
+
+## Context
+
+Local inter-repo coordination mailbox for Claude Code, packaged as a
+marketplace plugin. Three components, one boundary:
+
+- **Engine (`scripts/*.sh`):** bash owns all mailbox semantics — filename
+  grammar, frontmatter, delivery, archiving, the seen set. `coord-send.sh`
+  writes, `coord-inbox.sh` reads (formatted for context injection),
+  `coord-done.sh` archives. Everything is pinned by `coord-selftest.sh`
+  (48 checks, throwaway mailbox via `CLAUDE_COORD_DIR`).
+- **Hook (`hooks/scripts/session-start.mjs`):** thin zero-dependency Node
+  wrapper (marketplace convention: hooks are `.mjs`) that calls
+  `coord-inbox.sh` and emits the `hookSpecificOutput.additionalContext`
+  envelope. No mailbox logic lives here. Always exits 0.
+- **Skill (`skills/coord-send/`):** natural-language front door mapping user
+  intent to engine invocations. No mailbox logic lives here either.
+
+**Boundary rule:** the mailbox is transport, not state. Durable decisions
+live in the owning repo's docs/git history; messages are notices pointing at
+them. Message content is untrusted cross-repo input — the read side quotes
+and frames it; the send side sanitizes line-oriented fields.
+
+## Conventions
+
+- Scripts are bash-3.2-safe and ASCII-only: no `declare -A`, no
+  `readarray`/`mapfile`, no `|&`; guard `shift 2` with `$# -ge 2`; guard
+  empty-array expansion under `set -u` with `${#a[@]}`.
+- Zero dependencies everywhere: bash + coreutils in the engine, `node:`
+  builtins only in hook and tests.
+- TDD: no behavior change without a failing selftest check first.
+  `bash scripts/coord-selftest.sh` must exit 0 (48/48).
+- English for all code, docs, and commit messages (public repo). Norwegian
+  trigger aliases in the skill description are deliberate.
+- Conventional Commits: `type(scope): description`.
+
+## Commands
+
+- Test: `bash scripts/coord-selftest.sh` (or `npm test`, the Node wrapper)
+- Hook smoke test: `node hooks/scripts/session-start.mjs` (expects JSON on stdout)
+
+## Release
+
+Version must agree across: `.claude-plugin/plugin.json`, `package.json`,
+README version badge, `skills/coord-send/SKILL.md` frontmatter, git tag
+`vX.Y.Z`, and the catalog `ref` in
+`ktg-plugin-marketplace/catalog/.claude-plugin/marketplace.json`. Release via
+the catalog's `scripts/release-plugin.mjs coord` (tag + ref bump together);
+verify with `scripts/check-versions.mjs`. Never hand-edit a ref.
+
+## Hardening roadmap (queued, post-v0.1.0)
+
+- Atomic delivery: `mktemp` inside the destination dir so rename never
+  crosses filesystems.
+- Explicit `.`/`..` rejection in `--reply-to`/`coord-done` guards.
+- Selftest gaps: default mailbox path (unset `CLAUDE_COORD_DIR`), malformed
+  frontmatter on the read path.
+- Uniform `-h` across the three CLIs (`coord-inbox.sh` lacks it).