feat: add configurable agent ID support

[bmobytes]

Summary

This PR adds support for configuring which OpenClaw agent the integration communicates with, rather than always defaulting to the implicit main agent.

The OpenClaw gateway already supports routing requests to specific agents via the x-openclaw-agent-id header. This PR wires that capability through the full HA integration stack.

Changes

const.py

• Added CONF_AGENT_ID = "agent_id"
• Added DEFAULT_AGENT_ID = "main"
• Added ATTR_AGENT_ID = "agent_id" (for service calls)

api.py

• OpenClawApiClient.__init__ now accepts agent_id: str = "main"
• _headers() always includes x-openclaw-agent-id (gateway is unaffected when value is "main")
• async_send_message and async_stream_message each accept an optional per-call agent_id override

config_flow.py

• Manual setup step — new optional Agent ID text field (defaults to "main")
• Options flow — Agent ID field appears at the top of the options form so it can be changed at any time via Settings → Integrations → OpenClaw → Configure

__init__.py

• async_setup_entry reads agent_id from options (preferred) or config data, and passes it to the API client
• SEND_MESSAGE_SCHEMA gains an optional agent_id field so individual automation service calls can address a specific agent, overriding the configured default

services.yaml

• send_message service now documents the new optional agent_id field

strings.json / translations/en.json

• Labels added for the new agent_id option in both the manual config step and options flow
• Service field translation added

Behaviour

Scenario	Behaviour
agent_id not set / set to "main"	Identical to current behaviour
agent_id set to "wish-bmo" in options	All messages routed to that agent via x-openclaw-agent-id: wish-bmo
agent_id: "other-agent" in send_message service call	Per-call override, all other calls unaffected

Testing

1. Install updated integration
2. Go to Settings → Integrations → OpenClaw → Configure
3. Change Agent ID from main to another agent name
4. Send a message via the Lovelace card or openclaw.send_message service — it should route to the configured agent
5. Optionally pass agent_id in a service call to override per-message

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Note: This PR was implemented with assistance from an AI coding agent (BMO) and reviewed by a human before submission. It is a bot-assisted contribution that has undergone human review.

[techartdev]

Hi @bmobytes !
Thank you for the contribution. I have pushed the changes directly into main as I had to do some tweaks in order to make reload possible each time agent is changed. Otherwise user must do it manually and may forgot to do it, which will result in agent not being changed. I will merge the PR being now empty so it will count your contribution.