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

[jorisjonkers-dev-agents]

Follow-up from the lint sweep (PR #5). 39 functions — carve out lease, metadata, segment IO, and storage-stats stores while preserving the shared lock namespace and atomic file writes. 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): TranscriptStore.kt is 646 lines with @Suppress("LargeClass", "TooManyFunctions"). It handles lease management, metadata, segment I/O, and storage-stats in a single class, all sharing a lock namespace and atomic file writes.

Expected split (per issue):

• Lease store: lease acquire/renew/release
• Metadata store: per-session metadata read/write
• Segment I/O store: append/read transcript segments, atomic writes
• Storage-stats store: quota / usage metrics

The shared lock namespace must be preserved across all collaborators (pass a shared lock registry or use a wrapper).

Acceptance criteria:

• @Suppress("LargeClass", "TooManyFunctions") removed from TranscriptStore.kt.
• Atomic write guarantee preserved (confirmed by tests with concurrent writers).
• Requires a live tmux/container test environment.

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