docs: resolve OQ-TN-14 — unix in via local, stdio out (alktty owns process stdio)

Split the last open substrate-placement question:

- Unix: ships with the local feature v1 (dial_unix — same halves
  shape as TCP; the wire enum already carried unix per ADR-001;
  the params task's schema list includes all three values)
- Stdio: OUT of scope — a spawned process's stdin/stdout/stderr IS
  alktty's pipe mode (LocalTtyBackend + tokio::process + Stdio::piped,
  alktty tty-local.md): three multiplexed logical streams + the
  exit-code control chunk (alktty ADR-004) + signal forwarding
  (REQ-TTY-02). A stdio bridge here would be alktty's runner mode
  with the terminal stripped out — a strictly worse duplicate that
  also drops the semantics that matter (a byte tunnel has neither
  exit codes nor signals). Remote command execution composes via
  alktty on the same channels substrate.

Updated: open-questions.md OQ-TN-14 (resolved), overview.md feature
gate + deps + OQ summary, producer.md OQ ref, OQ-TN-10 promotion
(#3 split), phase-0-findings + both POC summaries' resolution notes,
params task (schema enum includes unix), local-socket-halves task
(unix ships, stdio does NOT — with the composition rationale),
oq-tn-14-tracker task repurposed (boundary-maintenance: re-opens only
if a consumer needs stdio-without-process-semantics — which would
need its own ADR, or if the alktty/alktunnels boundary needs
sharpening).

Verified: taskgraph valid (12 tasks, no cycles)

tasks/tunnels/local-socket-halves.md

@@ -47,6 +47,10 @@ pub struct UdpAssociateHalves { sock: Arc<UdpSocket> }  // the POC's associate s
   `poll_recv` into a too-small caller buffer must fail loud
   (OQ-TN-13's resolved posture — ADR-003). The codec wraps at this
   boundary; the pump never sees UDP specifics.
+- **Unix**: `dial_unix` ships (OQ-TN-14 resolved — same halves shape
+  as TCP). **Stdio does NOT** — a spawned process's stdio is alktty's
+  pipe mode (exit codes + signals are tty semantics, not tunnel
+  semantics); remote command execution composes via alktty, not here.
 - **Error mapping** to `TunnelEstablishError` variants (the POC's
   `Into<EstablishmentError>` path).
 - `TargetHandle` re-used from producer.rs (the halves type lives in