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)

docs/architecture/wire.md

@@ -35,6 +35,10 @@ this document is the normative WHAT.
   (NOT an address; the producer's registry maps it to its backing).
 - `substrate`: string, required, enum `["tcp", "udp", "unix"]` — the
   extensible discriminator (ADR-001); selects the data-plane framing.
+  The schema ships with all three values in v1 (unix has an
+  implementation path — OQ-TN-14). An older producer rejecting a
+  newer substrate value is the SSH "unknown channel type" posture —
+  loud, not silent.
 - `input_schema` pins both as required strings; unknown substrate
   values fail schema validation (typed error — loud, not silent).
 - Extensibility: a new substrate is a new enum value, not a format