fix(claude): drop the settings panel's Reasoning Effort Level control

The Claude Code panel offered a Reasoning Effort Level dropdown — Default, Low,
Medium, High, Extra High — that wrote a root-level `effortLevel` into the
agent's native config JSON. Like the Codex control removed alongside it, it read
as the knob that decides how hard a turn thinks while the composer's own
per-session picker is what actually governs that, so the two could disagree with
nothing on screen to say which one won.

The control goes together with everything behind it: the change handler, the
draft field, the extraction that read the key back out of the config, and the
normalizer that folded upstream's "default" sentinel into the placeholder.

An `effortLevel` already written into a config is left alone. Every surviving
writer spreads the parsed config before touching the keys it owns, and the
save-time diff compares config text against config text rather than rebuilding
the object from the fields the panel knows about, so a key nothing on screen
represents any more is carried through untouched. The native JSON editor below
is where it gets set by hand.