Remove the built-in todo tooling (todowrite/todoread) #182

Decision

Remove the built-in todo tooling. Callers who want plan tracking can add a custom tool (config/plugin custom tools), or tell the agent to keep a plan file. This matches the reference design: pi ships no built-in todo tool; it exists only as an example extension that users load explicitly.

Scope

Delete these from packages/cli:

• src/tool/todo.ts, src/tool/todowrite.txt, src/tool/todoread.txt
• src/session/todo.ts
• their registration and gating in src/tool/registry.ts, including the headless todo gating feat(run): drop todowrite from the default headless tool set (#155) #164 added
• todo references in:
  • src/agent/agent.ts (permission defaults)
  • src/tool/task.ts (subagent denies)
  • src/acp/agent.ts (todo-to-plan mapping; ACP clients no longer get plan entries, which is accepted)
  • src/cli/cmd/github.ts, src/cli/cmd/agent.ts, src/cli/cmd/run.ts
  • src/storage/json-migration.ts
  • src/config/config.ts
• mentions of TodoWrite in the base prompts (session/prompt/anthropic*.txt and any others)
• the related tests

If src/flag/headless-run.ts (#164) has no consumer left afterwards, delete it too and note that on #152.

Compatibility (explicit, tested)

• Config and permission keys naming todowrite/todoread (e.g. tools: { todowrite: false }, permission.todowrite) still load and do nothing.
• Stored todo data from older sessions is ignored, and --continue on a session whose history contains todowrite tool parts still works (test with a fixture).
• Leave generated SDK types to Bump @hey-api/openapi-ts to >=0.97.3 and regenerate @aictrl/sdk (GHSA-hhx9-57xq-r5rw) #166 unless typecheck forces a change.

Acceptance

• git grep -n -i "todowrite\|todoread" packages/cli/src finds only the compat handling.
• Typecheck and tests pass.
• The net diff is strongly negative.