Centralize session status policy

[ThomasK33]

What to build

Deepen the session lifecycle status rules into a shared status-policy module. The module should answer questions such as whether a session is terminal, active, commandable, live-host eligible, or offline-replay eligible, and the existing lifecycle, inspect, and gc paths should use that module without changing public CLI JSON or human output.

Acceptance criteria

• A shared status-policy module covers every SessionStatus value with assertions or exhaustive checks.
• src/host/lifecycle.ts, src/cli/commands/inspect.ts, and src/cli/commands/gc.ts use the shared policy instead of local status lists for terminal/active/offline decisions.
• Existing lifecycle, inspect, and gc behavior remains unchanged, including stale-host reconciliation and destroyed-session handling.
• Targeted tests cover the policy module directly and the existing inspect, gc, and lifecycle integration tests still pass.

Blocked by

None - can start immediately.

[ThomasK33]

This was generated by AI during triage.

Agent Brief

Category: enhancement
Summary: Centralize session lifecycle status decisions behind a shared policy module.

Current behavior:
Session lifecycle status rules are repeated across several command and host paths. Different modules each decide whether a session is terminal, active, commandable, live-host eligible, or offline-replay eligible. This makes status semantics easy to drift when a session state is added or reinterpreted.

Desired behavior:
Session status decisions should be expressed through one shared policy module that covers every SessionStatus value. Callers should use that policy for lifecycle, inspection, garbage collection, and running-session command decisions while preserving the existing public CLI JSON, human output, and storage behavior.

The change should be behavior-preserving. It should make the status taxonomy explicit and exhaustively checked, not change what any existing command returns.

Key interfaces:

• SessionStatus — every status value should be classified by the shared policy.
• Session status policy functions — should answer questions such as whether a session is terminal, active, running/commandable, live-host eligible, offline-replay eligible, and safe for garbage collection.
• Session lifecycle and command flows — should continue to use the same public error codes and result shapes when sessions are missing, running, terminal, destroyed, or stale.

Acceptance criteria:

• A shared session status policy covers every SessionStatus value with exhaustive checks or assertions.
• Lifecycle, inspect, garbage collection, and running-session command paths use the shared policy instead of local status lists where the decision is not command-specific.
• Public CLI JSON and human-readable output remain unchanged for existing lifecycle, inspect, garbage collection, and command-state scenarios.
• Tests cover the shared policy directly.
• Existing targeted lifecycle, inspect, garbage collection, and command-state tests still pass.

Out of scope:

• Adding, removing, or renaming SessionStatus values.
• Changing public CLI JSON contracts, protocol schemas, artifact formats, or event-log behavior.
• Refactoring unrelated command validation such as resize dimensions, signal names, key encoding, or text/file input resolution.