Remove the dead TodoWrite rendering subsystem from the CLI

[youknowriad]

Related issues

• Related to #

How AI was used in this PR

Used Claude Code to trace TodoWrite end-to-end — confirming via buildAgentTools and the pi runtime that it is not a registered tool, mapping every reference across the CLI TUI, and removing the wiring plus the state/tests it orphaned.

Proposed Changes

TodoWrite is not a tool the agent can call. buildAgentTools (apps/cli/ai/runtimes/pi/index.ts) constructs the full tool list explicitly — Read/Write/Edit/Bash/Grep/Glob/Ls plus the Studio tools, ask-user, and skill — and the pi runtime ships no todo built-in. So a TodoWrite call always fails with "Tool not found", and the CLI's todo diff/render machinery (which only does anything on a successful TodoWrite) has been dead since the pi-coding-agent migration (#3360) stopped surfacing synthetic todo messages. A code comment in the deleted todo-stream.ts already noted that the runtime "no longer surfaces" the todo message type.

This removes that unreachable subsystem: the todo diff engine, the snapshot rendering, the per-instance todo state on the CLI UI, and the now-unused display label. If a model still emits a stray TodoWrite out of habit, it now renders through the generic tool path (a normal "tool not found" line) instead of dedicated dead code — no user-facing behavior is lost, since the dedicated path could only ever render the same failure.

This is pure dead-code removal: no behavior that was actually reachable changes. Scope was deliberately limited to TodoWrite and the state/tests it orphaned; no broader dead-code sweep was done.

Companion to the annotate-workflow PR (#3870), which stops instructing the agent to call TodoWrite. The two are independent — this one removes the rendering, that one removes the prompt instructions.

Testing Instructions

• npm run cli:build && npm run typecheck — passes.
• npm test -- apps/cli/ai/tests/ui.test.ts — all 12 tests pass (the removed lines were leftover state setup, not todo assertions).
• Optional manual check: run the CLI agent and confirm tool rows (Read/Edit/Bash/Skill/etc.) still render correctly under their matching rows.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[youknowriad]

This is legacy handling of the todo tool that don't exist anymore.

[wpmobilebot]

📊 Performance Test Results

Comparing a9f81a5 vs trunk

app-size

Metric	trunk	a9f81a5	Diff	Change
App Size (Mac)	2355.89 MB	2355.88 MB	0.01 MB	⚪ 0.0%

site-editor

Metric	trunk	a9f81a5	Diff	Change
load	1612 ms	1627 ms	+15 ms	⚪ 0.0%

site-startup

Metric	trunk	a9f81a5	Diff	Change
siteCreation	8005 ms	8047 ms	+42 ms	⚪ 0.0%
siteStartup	3917 ms	3911 ms	6 ms	⚪ 0.0%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)