refactor(settings): rename 'llm' agent_kind discriminator to 'openhands' and LLMAgentSettings to OpenHandsAgentSettings

[simonrosenberg]

Background

The discriminated union introduced in v1.17 uses agent_kind: 'llm' | 'acp' to distinguish agent types. Both names are misleading:

• 'llm' describes the mechanism (uses an LLM), not the agent type. The correct semantic is 'openhands' — the built-in OpenHands agent, as opposed to an external ACP agent.
• LLMAgentSettings has the same problem — it should be OpenHandsAgentSettings.

Changes needed

1. AgentKind = Literal["llm", "acp"] → Literal["openhands", "acp"]
2. LLMAgentSettings.agent_kind: Literal["llm"] → Literal["openhands"]
3. All variant="llm" section/field metadata → variant="openhands"
4. Tag("llm") in AgentSettingsConfig union → Tag("openhands")
5. _agent_settings_discriminator default fallback → "openhands"
6. class LLMAgentSettings → class OpenHandsAgentSettings (keep LLMAgentSettings as a deprecated alias)
7. Update AgentSettings deprecation notice to point to OpenHandsAgentSettings

Notes

• Requires a minor version bump (breaking change to the agent_kind field default and type).
• Backwards-compat: add "llm" → "openhands" coercion in _agent_settings_discriminator for one release cycle to allow stored settings to migrate gracefully.