docs: record how a plugin's .mcp.json reaches apm, and document the gitea prerequisites

A review of this PR concluded that MCP was not an apm primitive, that the
.mcp.json edit therefore did nothing, and that the declaration belonged in
plugins/gitea/apm.yml under dependencies.mcp. The first half was right about
the primitive and wrong about everything that followed.

MCP is a first-class apm primitive. But the .mcp.json route already reaches
it: apm resolves a plugin manifest in the order plugin.json,
.github/plugin/plugin.json, .claude-plugin/plugin.json, so the generated
Copilot manifest wins, its mcpServers string pointer is followed, and
.mcp.json is injected into the package's dependencies.mcp with ${VAR} env
references intact. Verified against the real remote: a git-sourced install of
plugins/gitea at this branch deploys the gitea server with both references
unexpanded. No code change is needed and none is made here.

Moving the declaration into plugins/gitea/apm.yml would have broken the
build. apm-audit-ci runs apm audit --ci inside every plugins/*/, so a declared
dependency arms lockfile-exists there, which then demands an apm.lock.yaml in
the package plus that package's whole deployed tree inside the package
directory: 93 missing deployed files and 79 drifted paths, measured.

So this commit documents rather than changes:

- AGENTS.md and docs/spec/architecture.md said .mcp.json was plugin-root
  material with no .apm/ source, true of .apm/ and read as 'apm has no MCP
  concept'. Both now state what .mcp.json is, how it reaches dependencies.mcp,
  and that a plugin's own apm.yml is the one place not to declare it.
- architecture.md also records the env-strip: apm pack inlines .mcp.json into
  .claude-plugin/plugin.json and its sanitiser drops env and headers blocks
  unconditionally, ${VAR} included. Inert under apm, which never reads that
  file, but a native Claude Code plugin install reads exactly it and would
  start the server with no credentials.
- README.md gains the go toolchain prerequisite and the two environment
  variables the server needs, with placeholder values only.
- LESSONS.md records both process failures, including that three scratch
  installs inverted the result by using local ./path dependencies, where apm
  skips the plugin normalisation that injects .mcp.json.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EmiHiknxqtZPEBnW7ujgNz

docs/spec/architecture.md

@@ -49,6 +49,13 @@ Two compilers produce the plugin roots you see in the tree:
 
 `.apm/` is the sole hand-edited authoring source for plugin content. An edit made in the flat mirror is discarded by the next sync and is reported as drift by the `check-plugin-content-sync` pre-push hook. Hand-authored material that is not an `.apm/` primitive — `README.md`, `docs/`, `bin/`, `sources.md`, `.mcp.json`, and per-plugin extras such as `plugins/git/config.example.json`, `plugins/gitea/references/` and `plugins/bin/evals/` — lives at the plugin **root** and is untouched by either compiler.
 
+`.mcp.json` is the one entry in that list that is still load-bearing for apm rather than merely ignored by it. MCP is a first-class apm primitive — `dependencies.mcp` sits beside `dependencies.apm` in the manifest schema, and apm tracks deployed servers in `apm.lock.yaml` under `mcp_servers`, `mcp_configs` and `mcp_config_provenance`. A plugin reaches that primitive indirectly. `apm pack` writes the string `".mcp.json"` into the generated `.github/plugin/plugin.json` as its `mcpServers` value, and on install apm resolves the plugin manifest in the order `plugin.json`, `.github/plugin/plugin.json`, `.claude-plugin/plugin.json` — so the Copilot manifest wins, the pointer is followed, and `.mcp.json` is injected into the package's `dependencies.mcp` with any `${VAR}` env references intact. Verified against the real remote: a git-sourced install of `plugins/gitea` deploys the gitea server with both env references unexpanded.
+
+Two consequences follow, and both have bitten already:
+
+- **Do not declare `dependencies.mcp` in a plugin's own `apm.yml`.** It is the schema-correct place and it breaks the build. The `apm-audit-ci` pre-push hook runs `apm audit --ci` inside every `plugins/*/`, so a declared dependency arms `lockfile-exists` there, which then demands an `apm.lock.yaml` in the package plus every file of that package's own deployed tree present inside the package directory. Measured on `plugins/gitea`: 93 missing deployed files and 79 drifted paths.
+- **`.claude-plugin/plugin.json` carries an env-stripped copy.** `apm pack` inlines `.mcp.json` there, and its sanitiser drops `env` and `headers` blocks unconditionally at any depth, `${VAR}` indirection included. That copy is inert under apm, which never reaches it, but a native Claude Code plugin install reads exactly that file and would launch the server with no credentials. Anything installed natively rather than through apm needs its MCP env supplied by the host.
+
 That immunity is positional, not by filename. Anything placed *inside* a mirrored directory is destroyed regardless of what it is: `sync_dir` runs `rm -rf "$dst"` before every copy, and `sync_hooks_json` does the same to `hooks/`. A hand-written `README.md` under `plugins/<name>/hooks/` or `plugins/<name>/skills/` is deleted by the next sync with no drift report, because a file with no `.apm/` counterpart is simply absent from the regenerated tree. This has already cost the repo one document — `plugins/kyberforge/hooks/README.md`, since restored to `plugins/kyberforge/docs/hooks.md`. Plugin-root documentation belongs in `docs/`.
 
 ## Governance layer