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

[jorisjonkers-dev-agents]

Follow-up from the lint sweep (PR #5). 29 functions + spawn/maintainTranscripts complexity — split session registry, lease renewal, transcript maintenance, and spawn workflow into collaborators with lifecycle tests. 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): AgentSessionManager.kt is 686 lines with @file:Suppress("TooManyFunctions", "LargeClass", "TooGenericExceptionCaught", "LongMethod", "CyclomaticComplexMethod"). Its responsibilities span: session registry (spawn/get/list/stop), lease renewal, transcript maintenance, and the tmux session spawn workflow.

Expected split (per issue):

• Session registry collaborator: spawn lifecycle, get/list/stop, active-session map
• Lease renewal collaborator: handles renewal background loop
• Transcript maintenance collaborator: tail/rotate/segment logic
• Spawn workflow collaborator: tmux exec, post-spawn health checks

Acceptance criteria:

• All @Suppress annotations removed from AgentSessionManager.kt.
• Each collaborator has its own unit/integration test.
• Requires a live tmux/container test environment (the issue notes this).

File: services/agent-gateway/src/main/kotlin/…/tmux/AgentSessionManager.kt