feat(server,web): add read-only session status and micro-UI

[theshwal]

Summary

• add a lightweight read-only GET /api/sessions/:id/status endpoint
• derive session status from existing session/EventStore state without introducing a new source of truth
• extend the existing running indicator with running, waiting, blocked, and completed states
• keep the implementation outside the LLM request path and preserve KV-cache-related state

Validation

• 47 targeted tests passing
• typecheck passing
• lint passing
• full pre-commit suite passing
• QA_PASS on 712210bf3034b46a555927406bbc5544a9d692fb
• INT_PASS on the same SHA

Scope

No polling, heartbeat, new event journal, retry/recovery mechanism, or LLM-generated status text is introduced.

RunningIndicator remains read-only and reuses the existing client-side session state.

Refs #200

AI-Enhanced Development

• AI Models: MiniMax-M3 for implementation; GPT-5.6 for planning, review, QA/INT coordination, and PR preparation.

[theshwal]

Blocked on upstream PR #230, which addresses the pre-commit/E2E instability tracked in #225.
The Windows-path fix for this PR is prepared and locally validated, but is intentionally not committed yet because the mandatory pre-commit hook is currently blocked by unrelated flaky test behavior.