feat: add per-BTW model and thinking overrides

[counterposition]

Summary

Adds /btw:model and /btw:thinking commands so BTW can run with a different model or thinking level than the main thread — useful for keeping BTW cheaper or faster without changing main thread settings.

New commands

/btw:model [<provider> <model> <api> | clear]

• No args: shows the current effective BTW model and whether it is inherited or overridden
• With values: sets a BTW-only model override
• clear: removes the override and returns BTW to inheriting the main thread model
• If the configured override has no credentials, BTW warns and falls back to the main thread model

/btw:thinking [<level> | clear]

• No args: shows the current effective BTW thinking level and whether it is inherited or overridden
• With a value: sets a BTW-only thinking override for normal BTW chat
• clear: removes the override and returns BTW to inheriting the main thread thinking level

Behavior

• Changing either override disposes the active BTW sub-session; the new settings apply on the next BTW prompt while preserving the hidden thread
• /btw:summarize uses the effective BTW model but always keeps thinking off
• Overrides are persisted as hidden session entries (btw-model-override, btw-thinking-override) and restored on session reload
• api field added to BtwDetails so seed messages correctly reflect which API each past exchange used

Tests

6 new tests covering:

• BTW-specific model and thinking overrides applied to prompts
• Summarize uses BTW model but forces thinking off
• Clearing overrides restores inheritance from the main thread
• Override state restored from session history
• Read-only show commands report inherited vs overridden state
• Fallback to main model when override has no credentials

All 47 tests pass.

Co-Authored-By: Oz oz-agent@warp.dev