Sessions: explicit lock hierarchy (SessionLocks) and documented lock order; disconnect during a running command returns quickly #923

Goal

The order in which a session's locks may be taken is enforced by the API and documented in one place, so new code cannot introduce a deadlock between SFTP, transport and the elevated channel.

Context

• A session holds about 20 locks: 6 tokio mutexes, 2 in the transport (crates/app-logic/src/session.rs:~403/~407), 9 std mutexes, 3 std RwLocks, plus atomics (SessionParts, session.rs:696-991). Nested: crates/app-shell/src/elevated_sftp.rs:51/195 (Arc<AsyncMutex<Option<_>>> inside a StdMutex<HashMap>).
• The order is only stated in comments: crates/app-logic/src/orchestration/remote_files.rs (SFTP before transport), crates/app-logic/src/start_directory.rs:157-163 (avoids the reverse on purpose), crates/app-shell/src/elevated_sftp.rs:414-420 (revocation outside the transport lock because of disconnect).
• crates/app-shell/src/elevated_sftp.rs:401-403 holds the transport lock up to PROBE_TIMEOUT during the elevation probe.
• Since Keep the file browser, terminal and disconnect usable while a long command runs #328, AI commands release the transport lock after start_exec (crates/app-logic/src/orchestration/action_exec.rs:2004-2016); only transports without split start keep it for the whole command.
• No std guard is held across .await (clippy await_holding_lock finds 0) – keep it so.

Requirements

• Introduce SessionLocks (in app-logic, next to the session parts after their split) that owns the async locks of a session and exposes acquisition methods only in the permitted order, e.g. sftp(), transport(), sftp_then_transport(); there is no method that takes transport and then SFTP. Elevated-channel slots get the same treatment (documented level).
• All production sites that take more than one of these locks use SessionLocks; single-lock sites may keep direct access through it.
• Elevation probe: bound the time the transport lock is held to the probe command start (split start like Keep the file browser, terminal and disconnect usable while a long command runs #328) where the transport supports it.
• Document the hierarchy (levels, which lock protects what, rules: no std guard across .await, revoke outside the transport lock) in docs/architecture.md, and link it from the comments that currently state the order.
• Add clippy::await_holding_lock = "deny" to the workspace lints if the lint table exists.

Acceptance Criteria

• SessionLocks exists; a compile_fail doctest shows that acquiring transport then SFTP through the API does not compile (or is not expressible).
• Test: disconnect while an AI command runs on a transport without split start (mock) returns within 2 s (ordering/virtual time), the command is cancelled.
• Test: disconnect during an elevation probe returns within 2 s.
• docs/architecture.md has the lock hierarchy section; the three comments link to it.
• Existing session, SFTP pairing (Spec 0086 A3), elevated revocation (Spec 0085) and cancel (Spec 0027) tests pass unchanged.
• Relevant automated tests pass.

Technical Notes

• Security-critical: session pairing (Spec 0086 A3), elevated channel revocation (Spec 0085), the execution path (.claude/skills/smart-ssh-coder/references/security-invariants.md). Review priority elevated.
• Builds on the split of SessionParts into sub-structures.

Spec impact: update 0086 (lock order as part of A3)

Related Work

• Blocked by app-logic: group SessionParts fields into AiConfig, SafetyState, ChatPersistence and TransportState #871 (SessionParts split).
• Keep the file browser, terminal and disconnect usable while a long command runs #328, Spec 0027, Spec 0085, Spec 0086