docs: add ADR-009 (open op's input is the negotiation); fix stale doc refs

Review of the 2026-09-05 session commits:

- the ADR-009 decision cited by 96692d3 and docs/reviews/001 was never
  written — added decisions/009-channels-open-op-is-the-negotiation.md
  (context: the L1 two-gate disagreement, alkcall 0.4.0/0.4.1
  prerequisites, producer/consumer design, consequences, door type)
- channels.rs module doc + register_openable doc still described the
  old wire-frame negotiation read (drive_session) — aligned with
  drive_session_pre_negotiated and the enforced input schema
- tty-adapter.md: session-driver section + ADR tables now reference
  ADR-009 and the pre-negotiated driver; overview.md ADR index row

Verification: cargo test 93 lib (default) / 136 (--all-features);
clippy -D warnings native + wasm clean; fmt clean; doc 0 warnings.

docs/architecture/overview.md

@@ -285,6 +285,7 @@ PTY allocation code. See [ADR-003](decisions/003-local-backend-placement.md).
 | Self-contained negotiation framing | [ADR-006](decisions/006-negotiation-framing-self-contained.md) | alktty implements its own length-prefixed framing; format coincides with alkcall's by convention, not by code reuse |
 | TTY inside channels (reversed) | [ADR-007](decisions/007-tty-inside-channels.md) | Historical: the two-mode TTY design (direct vs inside-channels); reversed by ADR-008/093 |
 | Channels pure channel multiplexing | [ADR-008](decisions/008-channels-pure-channel-multiplexing.md) | TTY always uses its 5-byte format; the channels layer carries it transparently in the payload (reverses ADR-007) |
+| The open op's `input` is the negotiation | [ADR-009](decisions/009-channels-open-op-is-the-negotiation.md) | The channels path carries no second negotiation frame; the open op's registry-validated `input` is the negotiation |
 
 ## Open Questions