refactor: replace context_token_budget with model context_window for context trimming

The per-bot context_token_budget column was unused (no frontend UI) and
has been removed. Context trimming now derives the budget from the chat
model's context_window setting, which is already configured per model.

internal/db/sqlc/models.go

@@ -35,7 +35,6 @@ type Bot struct {
 	DiscussProbeModelID    pgtype.UUID        `json:"discuss_probe_model_id"`
 	TtsModelID             pgtype.UUID        `json:"tts_model_id"`
 	BrowserContextID       pgtype.UUID        `json:"browser_context_id"`
-	ContextTokenBudget     pgtype.Int4        `json:"context_token_budget"`
 	PersistFullToolResults bool               `json:"persist_full_tool_results"`
 	Metadata               []byte             `json:"metadata"`
 	CreatedAt              pgtype.Timestamptz `json:"created_at"`