Let a session choose how hard it thinks

Output is about an eighth of what a session costs and thinking is nearly
all of it -- prose is ~1.5% of output tokens, measured over 27,015 requests
of this account's own transcripts -- so the level is the largest saving
available short of shortening the conversation itself.

Shaped like the working directory rather than like the model: the CLI's
only two setting control requests are `set_model` and `set_permission_mode`
(checked against the 2.1.258 binary), so `--effort` is read when the process
launches and cannot be asked of a running one. `set_session_effort` records
the level and stops the process; the next message or Start launches one that
has it. That is also why the picker is in the session settings dialog beside
Move, and not on the bar beside the model and the mode, which take effect
mid-turn.

`None` is a level in its own right -- the CLI's own default -- so the picker
can return to it, and a blank is normalized to it at the boundary rather
than stored as a level the CLI would reject.

Offered only where it means something: `DriverKind::takes_effort` reports
the capability and the phone leaves the row out entirely, rather than the
session-type branch this app does not have anywhere else. A llama session
would otherwise get a control whose only effect is stopping its process.

Verified on the emulator against the sandbox's fake CLI: the picker sets it,
the server reports it, and an echo session's dialog is unchanged.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

PLAN.md

@@ -147,12 +147,26 @@ turn.
 
 Spawn: `claude -p --verbose --input-format stream-json --output-format
 stream-json --permission-mode <mode>` in the chosen working directory, plus
-`--model`. Wire-format notes are pinned against CLI 2.1.237 in
-`session/claude.rs`'s module doc: permissions need the hidden
-`--permission-prompt-tool stdio` flag, AskUserQuestion answers ride
-`updatedInput.answers` keyed by question text, and `set_model`/`interrupt`
+`--model` and, where one has been chosen, `--effort`. Wire-format notes are
+pinned against CLI 2.1.237 in `session/claude.rs`'s module doc: permissions
+need the hidden `--permission-prompt-tool stdio` flag, AskUserQuestion answers
+ride `updatedInput.answers` keyed by question text, and `set_model`/`interrupt`
 are control requests.
 
+**The thinking level is settled at launch** (added 2026-09-04, because it is
+the largest saving available on a long session: output is about an eighth of
+what a session costs and thinking is the bulk of output, against the ~1.5% that
+is prose). The CLI's only two setting control requests are `set_model` and
+`set_permission_mode` -- checked against the 2.1.258 binary -- so there is no
+way to ask a running process to think differently. `set_session_effort` is
+therefore shaped like `set_session_cwd` rather than like `set_session_model`:
+it records the level and **stops the process**, and the next message or Start
+launches one that has it. It lives in the session settings dialog beside the
+working directory for that reason, not on the session bar beside the model and
+the mode, which do take effect mid-turn. `None` is a level in its own right --
+the CLI's own default -- so the picker can return to it; a level this app named
+as the default instead would be this app choosing one.
+
 **`--resume` only ever runs when nothing else has that session open.** That
 is the rule behind the import refusal, the single `ClaudeDriver::launch`
 entry point, and the `Exited` correction below; two CLIs on one session file