Add Claude Opus 4.7 model and reasoning effort setting

[leonvanzyl]

Summary

• Update default models to claude-opus-4-7 and claude-sonnet-4-6 with automatic legacy model migration (opus-4-6, sonnet-4-5 IDs auto-remap)
• Add configurable reasoning effort level (low/medium/high/xhigh/max, default xhigh) across the full stack
• Effort setting threaded through registry, settings API, all SDK clients (main agent, assistant chat, spec creation, expand project), and both UI settings views

Test plan

• Restart server and verify Settings page shows claude-opus-4-7 / claude-sonnet-4-6 model buttons
• Verify Reasoning Effort selector appears with 5 levels, xhigh selected by default
• Change effort level and verify it persists across page reloads
• Start an agent and verify logs show Reasoning effort: xhigh (or selected level)
• Verify legacy model IDs in existing DB settings auto-migrate on first read

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