Document history retrieval and model-directed context resets

[cbinhan]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

Document development support for active-branch conversation history retrieval and the opt-in AgentResetCondenser for OpenHands/software-agent-sdk#4916.

The guide and architecture pages explain settings-based tool assembly, direct Agent requirements, model-written handoff, ordinary file notes, batch/input protection, and a single outer summary rescue with the existing finite internal retries. Existing summarization remains the default. The new feature is explicitly marked unreleased.

SDK draft PRs:

• History retrieval: feat(sdk): retrieve condensed conversation history on the active branch software-agent-sdk#5555
• Model-directed resets (depends on the history change): feat(sdk): add opt-in model-directed context resets software-agent-sdk#5556

Validation: Mintlify strict build and broken-links checks passed; the three new configuration snippets initialized real SDK Conversations against the implementation; both linked example paths exist. Browser rendering of the documentation site was not tested.

Keep this draft until the SDK implementation is accepted. No generated API spec was changed.