Pi reads its prompt from argv (positional, see buildPiArgs) and never
expects interactive input, so the Pi backend previously left cmd.Stdin
nil. Under systemd, the resulting /dev/null character device has been
observed not to satisfy Pi's readable-side wait, leaving runs stuck in
"working" forever (#2188).
Attach an explicit StdinPipe and close it immediately after Start so the
child sees an EOF on a FIFO, matching the pattern already used by the
Claude, Codex, Hermes, Kiro, and Kimi backends. The fix is defensive on
the daemon side because Pi is mid-refactor and is not accepting issues
upstream; once Pi itself stops blocking on stdin, this close is still
correct (a closed pipe is a no-op for a process that does not read it).
Test asserts the structural invariant: a shell-stub `pi` inspects
/proc/self/fd/0 and only emits a valid event stream when stdin is a
FIFO. If a future change drops the StdinPipe and stdin reverts to
/dev/null (char device), the stub exits non-zero and the test fails.
The Pi backend hardcoded `--tools read,bash,edit,write,grep,find,ls` in
buildPiArgs. Pi's SDK treats --tools as a restrictive allowlist: only the
listed tools pass through `_refreshToolRegistry()`, silently filtering
out any user-installed extension tools registered via `pi.registerTool()`.
Omitting --tools makes Pi's `allowedToolNames` undefined, so the
`isAllowedTool()` filter becomes a no-op and all tools — built-in and
extension — are available. This matches Pi's standalone behavior.
Users who want to restrict tools can still pass --tools via custom_args
(it is not in piBlockedArgs).
Closes#2379
