docs(410): a packaging contract, so a second client is a manifest and an adapter (#4032)

Step 5 of milestone 410 "One owner per piece of guidance". The operator
wants any attempt to package Scribe for another agent client to find the
repo already in the right shape.

plugin/PACKAGING.md (linked from the README) states:
- what every client package shares: plugin/skills/ (verbatim), the /mcp
  endpoint and its in-band responses, the /api/plugin/* adapter endpoints
  (context, retrieve, prior-art, tool-rules, processes), and one fmcp_ key
- what each client adds: a manifest; hooks limited to timing and transport;
  optional commands; adapter static text that never copies a skill or the
  index
- the Claude Code adapter file by file, as the worked example
- how Agent Plugins 1.0 clients (Codex, Cursor, Copilot/VS Code, Kiro,
  ChatGPT) and Gemini CLI would map, marked researched-not-tested (#4023)
- four open questions for the second package: passing the key to the MCP
  server, whether two manifests can share one folder, hook parity, and
  where process skills go

Hook audit: every hook prints only server-provided text, status or outage
lines, the running version, or the compaction reload pointer. No guidance
copies, so nothing moved.

Guard: test_the_skills_reference_nothing_outside_their_folder fails on a
relative path upward or a reference to plugin/, hooks/, commands/ or a
manifest from inside a skill, with a companion test showing it can fail.
Plugin version minted.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

plugin/README.md

@@ -31,7 +31,8 @@ holding the one copy, not by switching it off.
 every MCP client and serves live state; the skills in `skills/` state every
 reflex in full and name no client; this plugin is the Claude Code adapter —
 hooks that deliver at the right moment, `/scribe:sync`, and the few things only
-Claude Code needs said (`hooks/scribe_static_context.md`).
+Claude Code needs said (`hooks/scribe_static_context.md`). Packaging Scribe for
+another client: see [PACKAGING.md](PACKAGING.md).
 
 ## Install