feat: channels-path establisher migration (alkcall 0.5.0 / ADR-049) — bump to 0.2.0

Adopt alkcall 0.5.0's channel-open establishment phase (ADR-049 —
review 006 E-01 + N-1) and migrate the channels-path semantic failures
per its §5 sequencing (alktty ADR-010).

- `register_openable` registers `channels/tty/sub` with an establisher
  (`register_openable_with_establisher`): full `NegotiateRequest`
  parse of schema-valid `input`, `carriage == "raw"`, non-empty `cmd`,
  backend lookup, and the ADR-050 ownership check run before the open
  reply; rejections are `channel:open_failed` with `details.reason`
  (`unknown_resource` / `handler_error` / `timeout`) — no phantom
  channel (the SSH contract holds consumer-visibly)
- `backend.allocate` deliberately stays in the pump handler:
  `Establishment` is payloadless so the `TtyHandle` cannot cross the
  establisher→handler boundary, and re-allocating would violate
  ADR-005's kill-on-Drop contract — `allocate_failed` remains the one
  in-band failure class on the channels path (pinned by test)
- `TtySessionError::ChannelsOpen` carries alkcall's typed
  `ChannelOpenError` (`#[from]`) instead of a flattened `String` —
  the N-1 fix at alktty's layer (breaking)
- channels-path semantic failures change shape from
  `NegotiationRejected` in-band frames to `channel:open_failed` call
  errors (breaking); the direct-ALPN path is unchanged
- `tty_open_spec()` gains a `description` (review 006 E-02) and an
  ErrorDefinition for `channel:open_failed` (ADR-016 — disclosed via
  services/schema)
- alkcall = "0.5.0"; version 0.2.0; ADR-010 + ADR-009 amendment +
  tty-adapter.md + CHANGELOG

Verification: cargo test (112 lib + integration), cargo test
--all-features (136), clippy --all-targets -D warnings (host + wasm),
fmt --check, cargo doc --no-deps clean; wasm32-unknown-unknown check
confirms the default crate stays wasm-clean.

docs/architecture/decisions/009-channels-open-op-is-the-negotiation.md

@@ -11,6 +11,16 @@ of the schema-validated `input` is now a client-visible
 `malformed_negotiation` error frame, not a silent teardown — see
 §"Parse-failure error frame (R4 amendment, 2026-09-05)".
 
+Amended 2026-09-06 (ADR-010): with alkcall 0.5.0 (ADR-049 — review 006
+E-01), the semantic-failure classes the R4 amendment reported in-band
+(malformed negotiation, unknown backend, ownership denial) move into
+the producer's establisher and resolve as `channel:open_failed` call
+errors — no phantom channel. The in-band error-frame path on this
+channel shrinks to one class: `allocate_failed` (the establisher
+cannot carry the allocated handle across — see ADR-010 §2). The R4
+tests that asserted the in-band frames for those classes now assert
+the call-error shape.
+
 ## Context
 
 Before this ADR, the channels path carried the negotiation twice. The