refactor: split oversized functions (update 1555L, runNormalMode 648L, New 502L)

Summary

Function lengths below are exact, measured via AST (not brace-counting). Each bullet should be its own PR, in the listed order — smallest and most mechanical first.

Sequence

1. pkg/kit/extensions_bridge.go:24 bridgeExtensions — 390 lines. Registers tools, commands, widgets, themes and shortcuts in one function. Split per registration concern. Low risk, mechanical.
2. cmd/root.go:973 runNormalMode — 648 lines. Sequential phases (logging, prefs, MCP config, kit options, session setup, TUI launch) that extract cleanly into phase functions. Then group the 47-field runModeDeps (cmd/root.go:1723) into ~3 sub-structs (bridge getters / static snapshot / action funcs).
3. internal/agent/agent.go:611 GenerateWithCallbacks — 434 lines. Split the streaming vs non-streaming fork into generateStreaming / generateSimple. Pairs naturally with the HasAnyCallback() helper.
4. pkg/kit/kit.go:1500 New — 502 lines. Private phase funcs (applyOptions, resolveModelConfig, initExtensions); public signature unchanged. Covered by kit_test.go, bare_test.go, viper_isolation_test.go.
5. internal/ui/model.go:1652 update — 1555 lines, ~99 case labels. Its own dedicated PR, nothing bundled. Delegate per-domain to handler methods living in the existing stream.go / message_nav.go / etc. Highest leverage, highest regression surface; lean on model_test.go (1377 lines), children_test.go, message_nav_test.go.

Other >100-line functions exist (70 total; internal/ui/input.go:275 Update 328, cmd/extension_context.go:40 328, pkg/kit/kit.go:2684 generate 295, internal/extensions/runner.go:177 normalizeContext 284) but are lower priority.

Constraints — these would reintroduce known pitfalls

• Do not convert extensions.Context (84 fields, internal/extensions/api.go:47) to an interface. Yaegi cannot synthesise interfaces at runtime (genInterfaceWrapper nil deref); the function-field design is deliberate.
• Do not refactor anything on the View / distributeHeight measurement path while splitting model.go — height must be measured through the same render path as View(), so changes there are high-risk and low-reward.
• Preserve the go appInstance.NotifyWidgetUpdate() async pattern; never introduce a synchronous prog.Send() inside Update().

For a first PR, start at pkg/kit/extensions_bridge.go:24 and inspect bridgeExtensions alongside its registration concerns. Keep the listed sequence and constraints in view; for the kit.New and UI work, use kit_test.go, bare_test.go, viper_isolation_test.go, model_test.go, children_test.go, and message_nav_test.go to verify behavior remains covered.