[codex] Refactor heartbeat slash command module

[roackb2]

Summary

Completes the M5 slash-command migration slice by moving heartbeat commands into core modules and isolating the TUI-only debug snapshot command outside core.

Changes include:

• Added src/core/commands/slash/modules/heartbeat/heartbeat-commands.ts for /heartbeat tasks, /heartbeat task, /heartbeat runs, /heartbeat run, and /heartbeat continue.
• Added heartbeat ports to SlashCommandExecutionContext and wired the TUI adapter to createFileHeartbeatTaskStore.
• Added src/cli/chat/commands/debug-snapshot-command.ts so /debug tui-snapshot is registered from the TUI layer rather than core.
• Reduced local-commands.ts to compatibility exports, composed hints, autocomplete, /help, and registry orchestration.
• Added direct unit coverage for heartbeat routing and the pure continuation prompt builder.

Impact

Heartbeat command behavior is now reusable through the core slash registry while keeping store access host-supplied. The TUI-only debug command is explicitly host-owned, which keeps core command modules focused on reusable domains.

Validation

• yarn test:unit src/__tests__/unit/core/slash-command-modules.test.ts
• yarn test:unit src/__tests__/unit/tui/local-commands.test.ts
• yarn test:unit src/__tests__/unit/tui/heartbeat-cli.test.ts
• yarn vitest run src/__tests__/integration/core/heartbeat-scheduler.test.ts
• yarn eslint
• yarn typecheck
• yarn test outside the sandbox, because the full integration suite needs localhost binding and normal git identity for temp repositories