fix(pty): keep the controlled prompt so persistent bash settles fast

tool-bash-persistent overwrote the backend's PS1, so terminal-bash prompt
readiness never matched and every send degraded to the 3.5s silence tier
(idleSilenceMs + handoffGraceMs) under production defaults.

The controlled PROMPT_COMMAND now re-asserts PS1 before every prompt, so an
in-shell override never survives to the next prompt. The tool initializes
with stty -echo alone and detects the no-end-marker fallback through the
seam's stdin_read wait reason instead of matching its own prompt text.

Tool calls drop from 7180/3560/3566 ms to 355/88/91 ms (spawn+init+echo,
echo, pwd; darwin, production defaults). The loader composition suite now
pins the fast path by pushing idleSilenceMs beyond the send bound, and a
real-PTY case proves PS1 self-healing.

Fixes #2585

packages/shell/tool-bash-persistent/README.md

@@ -33,7 +33,7 @@ Prefix-stable while the configured description and schema remain unchanged.
 
 #### What the model sees
 
-Commands share one shell per Agent, so cwd, exported variables, activated environments, functions, and background jobs persist across calls. Results exclude private completion markers and the shell prompt. A nonzero wrapped command appends `[exit code: N]`; a shell that exits before reporting that status instead appends `[shell exited: code N]`, `[shell killed by signal: SIG]`, or `[shell exited]` when the backend supplies neither, then resets and tells the model that the next call starts fresh. Long output keeps the earliest retained prefix plus a clipping notice. If the PTY has already dropped that prefix, the result says so explicitly instead of presenting a tail as complete output. Timeout returns bounded partial output, closes the uncertain shell, and reports the reset.
+Commands share one shell per Agent, so cwd, exported variables, activated environments, functions, and background jobs persist across calls. Results exclude private completion markers. When the shell reads stdin again without having printed the completion marker — after `exec`, an interrupt, or an interactive foreground child whose stdin wait the provider proves — the call returns the captured partial output, which can end with the backend's own prompt text. A nonzero wrapped command appends `[exit code: N]`; a shell that exits before reporting that status instead appends `[shell exited: code N]`, `[shell killed by signal: SIG]`, or `[shell exited]` when the backend supplies neither, then resets and tells the model that the next call starts fresh. Long output keeps the earliest retained prefix plus a clipping notice. If the PTY has already dropped that prefix, the result says so explicitly instead of presenting a tail as complete output. Timeout returns bounded partial output, closes the uncertain shell, and reports the reset.
 
 #### Token effect
 
@@ -46,5 +46,6 @@ Append-only tool results follow the reusable request prefix.
 ## Known Limitations and Deferred Work
 
 - The tool requires an owning Agent and a real PTY backend.
+- An interactive foreground child (for example a REPL) returns early with partial output only where the subprocess provider proves its stdin wait; elsewhere the call runs to `timeoutMs`.
 - Explicit `exit` and timeout discard shell state. Cancellation also resets and discards the result, even when a complete status marker is already observable; the next call starts a fresh shell.
 - Environment facts such as network access and package mirrors belong in the configured `description`, not this package's default.