fix: supplement agent model list with known Claude models

[taariq]

Summary

• Supplement the agent model selector with known Claude models that the CLI may not advertise
• The Claude Code CLI initialize response only reports a subset (Default, Sonnet, Haiku) -- Opus and other models were missing
• After parsing CLI-reported models, adds any missing known models: Opus 4.6, Opus 4.1, Sonnet 4.6, Sonnet 4.5, Sonnet 4, Haiku 3.5
• Existing CLI-reported models are preserved, duplicates are skipped
• Includes 2 new tests verifying supplement behavior

Test plan

• cargo test passes (5 tests including 2 new supplement tests)
• Start Claude Code agent session, open model selector
• Verify Opus 4.6, Opus 4.1, and full model list appear in dropdown
• Select Opus, verify agent uses it

Closes serenorg/seren-desktop#880

Taariq Lewis, SerenAI, Paloma, and Volume at https://serendb.com
Email: hello@serendb.com