refactor(AgentAttachHandler): split to remove TooManyFunctions/complexity suppressions

[jorisjonkers-dev-agents]

Follow-up from the lint sweep (PR #5). 21 functions + 89-line afterConnectionEstablished — separate durable-attach, live-attach, and input-handler collaborators with websocket close/error coverage. Goal: remove the localized @Suppress once split, with integration coverage. Requires a live tmux/container test env.

[jorisjonkers-dev-agents]

Ready to pick up

Current state (verified): AgentAttachHandler.kt is 605 lines with @Suppress("LargeClass", "TooManyFunctions"). Its afterConnectionEstablished method is 89 lines. Responsibilities: durable attach (resume after disconnect), live attach (new session), WebSocket input routing, output relay, and close/error handling.

Expected split (per issue):

• DurableAttachHandler: resume logic for reconnecting clients
• LiveAttachHandler: new connection setup
• InputHandler: WebSocket message routing / Claude input formatting
• The base AgentAttachHandler becomes a thin coordinator

Acceptance criteria:

• @Suppress("LargeClass", "TooManyFunctions") removed from AgentAttachHandler.kt.
• WebSocket close/error paths covered by tests (currently uncovered for edge cases).
• Requires a live WebSocket test environment.

File: services/agent-gateway/src/main/kotlin/…/ws/AgentAttachHandler.kt