fix(pi): read pi's composer rule so a pi turn can end

A pi session stayed "working" for good once a turn started. pi's braille
spinner trips the SPINNER_PATTERN fast path, which marks the pane working,
but only a composer glyph arms the idle confirmation and pi declared none,
so it fell back to Claude's `❯`, which pi never draws. Measured on beta136:
an errored turn stayed busy/isWorking for 3+ minutes after pi was back at
rest.

pi has no composer glyph. Measured on a live pi 1.1.0 pane (capture-pane
every 250 ms through a turn): its composer sits between two `─` rules, and
while a turn runs it rewrites the top rule as `── ⠏ Working ───` on every
frame. The registry entry now names the rule as the glyph that arms the
check and a spinner frame inside it as the working line.

The same glyph settles a reattached pi pane (a restored pane gets no launch
timer): tmux's reattach repaint carries `─`, which arms the confirmation.
The submit verifier reads the last rule, finds no prompt text and stands
down, so it can never press Enter on a pi pane.

Verified on an isolated instance from this branch: a real pi turn emitted
session:working then session:idle, and after a server restart the restored
pi pane went busy -> idle in about 3 s.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

src/session.ts

@@ -4479,7 +4479,7 @@ export class Session extends EventEmitter {
           : this._mux.capturePaneText?.(this._muxSession.muxName),
       sendEnter: () => this._mux?.sendInput(this.id, '\r'),
       // ⚠ NO fallback glyph here, unlike the screen-reading probe elsewhere in this file.
-      // Only claude and codex declare a promptGlyph; the other eight modes would fall back
+      // Only claude, codex and pi declare a promptGlyph; the other modes would fall back
       // to claude's `❯`, which is ALSO starship's default shell prompt (and pure's, and
       // spaceship's, and p10k lean's). On a shell session the line `❯ npm run build` sits
       // on screen for as long as the command runs, promptStillInComposer() reads that as