fix(desktop): Claude model picks + agent avatars after restart

[Chessing234]

Summary

• Claude Code agents honor the Edit Agent model picker via ANTHROPIC_MODEL at spawn
• Agents screen keeps custom avatars after restart by preferring the managed agent URL

Test plan

• just ci
• Set a Claude Code agent model ≠ global default → restart → ask which model it is
• Set a custom agent avatar → quit/relaunch Buzz → Agents screen still shows it

Closes #2692
Closes #2576

Made with Cursor

[bosoud]

Heads up: #2695 (opened earlier today, also fixing #2692) carries this same fix — model_env_var: Some(ANTHROPIC_MODEL) + the derived-key addition, with two extras worth keeping whichever PR moves forward:

• a shared ANTHROPIC_MODEL_ENV_KEY const in env_vars.rs referenced by the runtime entry, the derived-key list, and the existing literal in readiness.rs's provider-model mapping — three call sites, one definition, so the key can't drift;
• a regression test pinning the claude runtime shape itself (known_acp_runtime_exact("claude").model_env_var == Some("ANTHROPIC_MODEL"), provider still locked).

No attachment to whose lands — happy to close #2695 if you'd rather fold the const + readiness dedup in here (the avatar fix bundled in this PR is orthogonal either way), or rebase mine if you prefer to split. Maintainers' call.

[Chessing234]

Folded in the shared ANTHROPIC_MODEL_ENV_KEY + claude runtime shape regression from #2695. Avatar fix stays here; fine if you close #2695 as duplicate.