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/const.py

@@ -23,6 +23,7 @@ CONF_GATEWAY_TOKEN = "gateway_token"
 CONF_USE_SSL = "use_ssl"
 CONF_VERIFY_SSL = "verify_ssl"
 CONF_ADDON_CONFIG_PATH = "addon_config_path"
+CONF_AGENT_ID = "agent_id"
 
 # Options
 CONF_INCLUDE_EXPOSED_CONTEXT = "include_exposed_context"
@@ -36,6 +37,7 @@ CONF_VOICE_PROVIDER = "voice_provider"
 CONF_BROWSER_VOICE_LANGUAGE = "browser_voice_language"
 CONF_THINKING_TIMEOUT = "thinking_timeout"
 
+DEFAULT_AGENT_ID = "main"
 DEFAULT_INCLUDE_EXPOSED_CONTEXT = True
 DEFAULT_CONTEXT_MAX_CHARS = 13000
 DEFAULT_CONTEXT_STRATEGY = "truncate"
@@ -130,6 +132,7 @@ ATTR_SESSION_KEY = "session_key"
 ATTR_DRY_RUN = "dry_run"
 ATTR_MESSAGE_CHANNEL = "message_channel"
 ATTR_ACCOUNT_ID = "account_id"
+ATTR_AGENT_ID = "agent_id"
 ATTR_OK = "ok"
 ATTR_RESULT = "result"
 ATTR_ERROR = "error"