feat: rename the experimental todowrite tool to todo - #57481

Refs #42421

Renames the experimental todowrite tool, added in #56947, to todo. It shipped in v2.1.0 behind experimental.todo, so this is the cheapest time to change the name.

- todowrite({ todos: [...] })
+ todo({ todos: [...] })

• Name: the tool and its permission action are now todo. The input, the reply, and the flag are unchanged.
• Old calls still display: sessions from V1 migration or from today keep their todowrite calls. The TUI renders them as todo calls in the timeline, sidebar, and mini mode via canonicalToolName. Desktop/web hides both names until desktop support lands.
• No alias: a model that calls todowrite gets the normal unknown-tool error.
• V1 config: a permission.todowrite key maps to todo, alongside the existing bash→shell and task→subagent renames.
• V1 migration notice: now reads "The todowrite tool is no longer available. If todo is in your tool list, use it instead."
• Docs: updated to the new name.

Verification

• packages/core: bun typecheck passes. bun test test/v1-migration.test.ts test/tool-todo.test.ts test/config: 276 pass.
• packages/tui: bun typecheck passes. test/util/tool-display.test.ts and test/mini: 430 pass.
• packages/session-ui: rows-current.test.ts passes, 51 tests. It covers hiding both names.
• packages/schema and services/www: typecheck pass. check:generated shows no OpenAPI change.

Added by the opencode bot (AI-generated).

Related: #42421

Public API: renames the experimental tool and its permission action from todowrite to todo; V1 permission keys are mapped, but V2 permissions rules using todowrite need renaming