docs: add deploy_admin tool with opt-in flag to homelab-deploy plan

MCP exposes two tools:
- deploy: test-tier only, always available
- deploy_admin: all tiers, requires --enable-admin flag

Three security layers: CLI flag, NATS authz, Claude Code permissions.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/plans/nats-deploy-service.md

@@ -151,18 +151,35 @@ This provides immediate feedback on validation errors (bad revision, already run
 
 ### MCP Mode
 
-Runs as an MCP server providing tools for Claude Code:
+Runs as an MCP server providing tools for Claude Code.
-- `deploy` - Deploy to specific host, all hosts in tier, or all hosts with a role
-- `deploy_status` - Check deployment status/history
-- `list_hosts` - List available deployment targets
 
-The MCP server runs with limited credentials (test-tier only), so Claude can:
+**Tools:**
+| Tool | Description | Tier Access |
+|------|-------------|-------------|
+| `deploy` | Deploy to test hosts (individual, all, or by role) | test only |
+| `deploy_admin` | Deploy to any host (requires `--enable-admin` flag) | test + prod |
+| `deploy_status` | Check deployment status/history | n/a |
+| `list_hosts` | List available deployment targets | n/a |
+
+**CLI flags:**
+```bash
+# Default: only test-tier deployments available
+homelab-deploy mcp --nats-url nats://nats1:4222
+
+# Enable admin tool (requires admin NKey to be configured)
+homelab-deploy mcp --nats-url nats://nats1:4222 --enable-admin --admin-nkey-file /path/to/admin.nkey
+```
+
+**Security layers:**
+1. **MCP flag**: `deploy_admin` tool only exposed when `--enable-admin` is passed
+2. **NATS authz**: Even if tool is exposed, NATS rejects publishes without valid admin NKey
+3. **Claude Code permissions**: Can set `mcp__homelab-deploy__deploy_admin` to `ask` mode for confirmation popup
+
+By default, the MCP only loads test-tier credentials and exposes the `deploy` tool. Claude can:
 - Deploy to individual test hosts
 - Deploy to all test hosts at once (`deploy.test.all`)
 - Deploy to test hosts by role (`deploy.test.role.<role>`)
 
-Production deployments require admin credentials.
-
 ### Tiered Permissions
 
 Authorization is enforced at the NATS layer using subject-based permissions. Different deployer credentials have different publish rights: