feat: add configurable agent ID support

Allow users to configure which OpenClaw agent the integration communicates
with, instead of always defaulting to the implicit 'main' agent.

Changes:
- const.py: add CONF_AGENT_ID, DEFAULT_AGENT_ID='main', ATTR_AGENT_ID
- api.py: accept agent_id in constructor; _headers() now includes the
  x-openclaw-agent-id header on every request; async_send_message and
  async_stream_message accept an optional per-call agent_id override
- config_flow.py: expose agent_id as a text field in both the manual
  setup step and the options flow (Settings → Integrations → Configure)
- __init__.py: read agent_id from options/data and pass it to the API
  client; add optional agent_id field to the send_message service schema
  so automations can address a specific agent per-call
- services.yaml: document the new agent_id field on send_message
- strings.json / translations/en.json: add UI labels for the new option

The gateway routing header x-openclaw-agent-id is always sent; when
agent_id is 'main' (the default) the gateway behaviour is unchanged.
A per-call override on send_message takes precedence over the config.

Implemented with assistance from an AI coding agent (BMO) with human
review. Bot-assisted contribution reviewed by a human before submission.

custom_components/openclaw/translations/en.json

@@ -18,7 +18,8 @@
           "gateway_port": "Gateway Port",
           "gateway_token": "Gateway Token",
           "use_ssl": "Use SSL (HTTPS)",
-          "verify_ssl": "Verify SSL certificate"
+          "verify_ssl": "Verify SSL certificate",
+          "agent_id": "Agent ID"
         }
       }
     },
@@ -39,6 +40,7 @@
         "title": "OpenClaw Options",
         "description": "Configure context and tool-calling behavior.",
         "data": {
+          "agent_id": "Agent ID (e.g. main)",
           "include_exposed_context": "Include exposed entities context",
           "context_max_chars": "Max context characters",
           "context_strategy": "When context exceeds max",
@@ -126,6 +128,10 @@
         "attachments": {
           "name": "Attachments",
           "description": "Optional file attachments."
+        },
+        "agent_id": {
+          "name": "Agent ID",
+          "description": "Optional OpenClaw agent ID to route this message to. Overrides the configured default. Defaults to \"main\"."
         }
       }
     },