fix: local-bridge robustness, scope-gate order, sentinel contract; packaging/docs closeout (P8-P11, P2/P3/P15)

Closes review #003 (prepublish review for v0.1.0).

- P8: StdinSink::poll_shutdown parks an inflight reserve+send on a
  full channel (waker registered) — a stdin blast followed by EOF
  delivers the EOF instead of stranding it
- P9: five poisoned-lock .expect() sites -> unwrap_or_else(into_inner)
- P10: three thread-spawn .expect() sites -> TtyError::AllocFailed
- P5: tty:open scope gate runs before carriage/cmd/backend-lookup
  checks (no backend-name enumeration differential for unscoped ids)
- P11: recv_stdout terminates on the zero-length drained sentinel;
  the sentinel is no longer yielded as an item (doc was already the
  contract); stderr has no sentinel (doc noted)
- P2: exclude AGENTS.md + docs/plans/, drop dead Cargo.lock and
  docs/research/ entries (package list: 42 files, 659.2KiB)
- P3: AGENTS.md phase status (all five landed), ADR range 001..009
  (+ alktty-native ADR-009 in the mapping), alkcall guidance
  corrected to v0.4.x / pin "0.4.0"; architecture README ADR-009 row
  + landed-phase status
- P15: backend.rs doc typo; redundant tokio-stream dev-dep removed;
  NegotiationError::Io arm logs; set_identity failure logs;
  input_pump.abort() at session end; TtySessionError::Open carries
  the accept_bi StreamError (no io::Error flattening); borrowing
  deserialize in open_via_channels (no params.clone());
  error_response_bytes guards an "error" key in fields; trivial
  inline comments promoted/removed; plan-doc test counts + doc
  front-matter refreshed; session tests that raced session teardown
  under the abort change use a GatedBackend (exit held until
  released)

Verification: cargo test 104 lib / --all-features 147; clippy
(all-targets + wasm32) -D warnings; fmt; wasm check; doc 0 warnings;
publish dry-run OK.

docs/architecture/overview.md

@@ -2,8 +2,8 @@
 status: draft (ported from alknet 2026-08-17; alknet-tty → alktty,
   alknet-tty-local → alktty's `local` feature module, alknet/tty →
   alk/tty, alknet-core → alkcall::core, alknet-call → alkcall, ADRs
-  renumbered 052..093 → 001..008)
-last_updated: 2026-08-17
+  renumbered 052..093 → 001..009)
+last_updated: 2026-09-05
 ---
 
 # alktty — Overview