feat(acp): add opt-in Coven runtime bridge

Add the opt-in Coven ACP runtime bridge as a bundled extension while keeping ACPX as the default path.

Security hardening included before merge:
- fail closed by default instead of silently falling back;
- bounded health/socket requests and daemon response sizes;
- fixed Coven socket trust anchor and symlink/path validation;
- reject untrusted harness/session/event ids before exposing them;
- sanitize daemon-controlled terminal/status/error strings;
- use incremental event polling with bounded dedupe state;
- clean up launched Coven sessions before fallback when daemon ids are invalid.

Validation:
- pnpm test extensions/coven/src/config.test.ts extensions/coven/src/client.test.ts extensions/coven/src/runtime.test.ts
- pnpm check:changed
- GitHub CI green on a64eac20b9
- Greptile Review green

docs/tools/acp-agents-setup.md

@@ -186,6 +186,77 @@ Override the command or version in plugin config:
 
 See [Plugins](/tools/plugin).
 
+## Optional Coven backend
+
+OpenClaw can also register a bundled, opt-in `coven` ACP backend for operators
+who want ACP coding sessions supervised by a local [Coven](https://github.com/OpenCoven/coven)
+daemon instead of launched directly through ACPX.
+
+This is intentionally an extension, not a core runtime path:
+
+- the default ACPX backend stays unchanged for normal installs;
+- Coven has its own daemon, socket, session store, harness mapping, and project
+  boundary model;
+- the bridge can be enabled, disabled, configured, and reviewed independently
+  through the plugin system; and
+- OpenClaw remains responsible for ACP session routing, chat bindings, task
+  state, and fallback policy while Coven owns harness supervision.
+
+Minimal opt-in config:
+
+```json5
+{
+  acp: {
+    enabled: true,
+    backend: "coven",
+    defaultAgent: "codex",
+  },
+  plugins: {
+    entries: {
+      coven: {
+        enabled: true,
+        config: {
+          // Optional. Defaults to ~/.coven. Environment variables are not used for this trust anchor.
+          covenHome: "~/.coven",
+          // Optional. Defaults to <covenHome>/coven.sock; overrides must resolve to that path.
+          socketPath: "~/.coven/coven.sock",
+          // Optional. Defaults to false; enable only when direct ACP fallback is acceptable.
+          allowFallback: false,
+          // Optional. Used only when allowFallback is true.
+          fallbackBackend: "acpx",
+        },
+      },
+    },
+  },
+}
+```
+
+When selected, OpenClaw checks Coven daemon health over the configured Unix
+socket before launching. A successful launch creates a Coven session and records
+the Coven session id in the ACP runtime handle. If the health check or launch
+fails, OpenClaw fails closed by default so `acp.backend="coven"` cannot silently
+downgrade to direct ACP execution. Set `allowFallback: true` only when direct
+ACP fallback is an explicit, acceptable operator choice.
+
+For path safety, `~` in `covenHome` and `socketPath` expands to the current
+user home directory, and configured Coven paths must be absolute after that
+expansion. OpenClaw rejects workspace-relative Coven daemon paths because the
+daemon socket is a local user trust anchor, not repository-controlled state.
+`socketPath` must resolve to `<covenHome>/coven.sock`; OpenClaw does not allow
+arbitrary Coven socket filenames because the daemon socket is the local trust
+anchor. Keep `covenHome` owned by the OpenClaw user and private (`0700`);
+OpenClaw rejects symlinked, shared-accessible, shared-writable, or non-socket
+Coven socket paths before connecting. The Coven backend currently requires Unix
+socket validation and fails closed on Windows rather than trusting a socket path
+whose owner and permissions cannot be validated by this plugin.
+
+The default harness mapping sends known ACP agent ids such as `codex`, `claude`,
+`gemini`, and `opencode` to explicitly authorized Coven harness ids. Unknown
+ACP agent ids are rejected instead of being forwarded as harness names. Override
+`plugins.entries.coven.config.harnesses` only when your local Coven install uses
+custom harness names, and keep `acp.allowedAgents` aligned with the intended
+chat-exposed harness set.
+
 ### Automatic dependency install
 
 When you install OpenClaw globally with `npm install -g openclaw`, the acpx