CB-527: ship the bridge as an installable Claude Code plugin

The orchestration contract had no distributable form. Every consuming project
hand-copied a block of CLAUDE.md and hand-wrote an .mcp.json, and we keep a
script whose only job is to notice those copies drifting apart. A plugin is
versioned, installed once, and updates in place.

Ships no credentials, deliberately: every secret is referenced by environment
variable NAME and the value never enters a file, which is what makes the
artifact safe to publish. The setup skill states the two rules that are easy to
get wrong for the right-sounding reasons — the PR token must not be able to
merge (a worker opens, the primary gates), and ANTHROPIC_BASE_URL must never be
set by setup, because mounting the bridge must not move a session off
subscription.

The plugin root is plugin/, not the repo root. An installed plugin's .mcp.json
is a committed file, while this repo's root .mcp.json is local-only and
--skip-worktree; rooting the plugin at the repo would commit the primary's IDE
servers and hand them to every worker — the exact failure CB-525 exists to
prevent.

Scope is client-side setup only. herdr and bridged stay separate services with
their own lifecycles, and the skill refuses to install them rather than guess.
It also refuses to accept /healthz as proof: health reports only that the daemon
can reach herdr, and CB-521 showed it staying green while every spawn failed, so
verification ends with a real spawn.

Both manifests pass `claude plugin validate --strict`.

.claude-plugin/marketplace.json

@@ -0,0 +1,18 @@
+{
+  "name": "claude-bridge",
+  "description": "Tooling for orchestrating a fleet of delegated coding agents through the bridged MCP gateway.",
+  "owner": {
+    "name": "LTMS"
+  },
+  "plugins": [
+    {
+      "name": "claude-bridge",
+      "source": "./plugin",
+      "description": "Make a project bridge-ready: mount the bridged MCP gateway and apply standard Claude Code settings so a session can orchestrate delegated workers. Ships no credentials.",
+      "version": "0.1.0",
+      "author": {
+        "name": "LTMS"
+      }
+    }
+  ]
+}