`attach_file` was registered for plan-mode turns but never added to
`builtinPlanToolAllowed`, so the per-turn `ActiveTools` allowlist
filtered it out and calls failed with `Tool not active in this turn:
attach_file`. This was an omission rather than a deliberate block — the
tool (#24280) landed shortly after plan mode (#24236) and no subsequent
edit to the allowlist picked it up.
Add `attach_file` under the `isRootChat` case, matching how other
artifact-producing tools (`propose_plan`, `write_file`, `edit_files`)
are gated. The tool only reads from the workspace and writes to
chat-attachment storage, so it preserves plan mode's invariant of not
making implementation changes to the workspace. Subagents in plan mode
remain restricted to the minimal read-only surface.
Ethan
9444eddf4e