setup: offer the agent's on-demand camera mode, defaulting to on_demand (#179)

[Adityakk9031]

Closes #179.

Problem

-P images=on_demand (#173, #175) allows the agent policy to issue take_pic tool calls for frames on-demand instead of attaching all camera frames to every observation. While this is cheaper and preferable for real rigs, it was only discoverable via the plugin README. inspect-robots setup is where operators configure defaults, but it previously did not prompt for agent policy options.

Solution

1. Setup Wizard (_setup.py):
   • Added _valid_images_mode() validator ("on_demand" / "always").
   • Gated a prompt on configured_policy == "agent" that suggests on_demand (explaining the token savings vs model responsibility tradeoff) while preserving any existing images configuration in [policy.args] on setup re-runs.
   • Updated _render_config() to render managed [policy.args] blocks alongside [defaults] and [embodiment.args] while preserving any unmanaged policy args.
2. Documentation:
   • Updated plugins/inspect-robots-agent/README.md to note that inspect-robots setup suggests on_demand while preserving the library default always.
   • Updated docs/guide/cli.md and docs/guide/quickstart.md wizard descriptions.
3. Tests (tests/test_setup.py):
   • Added unit test test_render_config_renders_policy_args.
   • Added integration tests for prompt defaults (on_demand), explicit selection (always), and config preservation on setup re-runs.

Verification

• ruff check . — All checks passed.
• ruff format --check . — All 108 files formatted.
• pytest tests/test_setup.py — 105 passed, 16 skipped (POSIX/symlink-gated tests skipped on Windows as expected).

[Adityakk9031]

@jeqcho have a look

[jeqcho]

Thanks @Adityakk9031! CI on this branch is green, but it now has merge conflicts with main. Could you rebase and resolve? We'll review right after. Thank you! 🙏