stores: persist only non-default agent settings

[enyst]

Summary

• Switch agent_settings.json persistence to a CLI-owned, versioned schema instead of dumping the full SDK Agent model.
• Persist only user-facing fields (model, api_key, optional base_url, and memory_condensation_enabled when non-default) and reconstruct a runtime Agent using current SDK + CLI defaults.
• Maintain backwards compatibility by loading legacy full-Agent dumps and migrating them in-place.

Why

Storing a full Agent snapshot pins SDK defaults and makes saved configs large and brittle across SDK releases. Persisting only explicit overrides keeps configs smaller and allows defaults to evolve safely.

Testing

• make format
• make lint
• make test

Fixes #302

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🚀 Try this PR

uvx --python 3.12 git+https://github.com/OpenHands/OpenHands-CLI.git@openhands/agent-settings-non-default

[github-actions]

Coverage Report •

File	Stmts	Miss	Cover	Missing
openhands_cli/stores
agent_store.py	225	21	90%	106, 124, 127, 166, 308, 363–364, 367, 392, 563–564, 566, 573, 580, 582, 591, 594–595, 598–599, 601
TOTAL	6552	911	86%