Add configurable allowed hosts for MCP remote access (KB_MCP_ALLOWED_HOSTS)

The MCP SDK's DNS rebinding protection rejects remote clients with 421
when the Host header isn't in the allowlist. Add KB_MCP_ALLOWED_HOSTS env
var (comma-separated IPs/FQDNs) to configure additional allowed hosts
while keeping localhost always permitted.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

openspec/changes/mcp-allowed-hosts/tasks.md

@@ -0,0 +1,18 @@
+## 1. Configuration
+
+- [x] 1.1 Add `KB_MCP_ALLOWED_HOSTS` to `mcp/config.py` — read from env, default empty string
+- [x] 1.2 Add host-parsing helper that splits the comma-separated value, strips whitespace, and filters empty entries
+
+## 2. Transport security
+
+- [x] 2.1 Build `TransportSecuritySettings` in `mcp/server.py` — merge localhost defaults with parsed `KB_MCP_ALLOWED_HOSTS`, derive allowed origins from allowed hosts
+- [x] 2.2 Pass `transport_security=` to the `FastMCP()` constructor
+
+## 3. Compose files
+
+- [x] 3.1 Add `KB_MCP_ALLOWED_HOSTS=${KB_MCP_ALLOWED_HOSTS:-}` to the kb-mcp environment block in `compose.cpu.yaml`, `compose.nvidia.yaml`, and `compose.rocm.yaml` with a comment explaining the format
+
+## 4. Verification
+
+- [x] 4.1 Test: unset `KB_MCP_ALLOWED_HOSTS` — confirm localhost connects, remote host gets 421
+- [x] 4.2 Test: set `KB_MCP_ALLOWED_HOSTS` to the server IP — confirm remote host connects successfully