feat(web): add session unarchive operation

[seekskyworld]

Problem

Implements the reversible archive-management slice of #347. The Web API could archive sessions but offered no way to restore one.

Value

Archived Session metadata can be restored without deleting or rewriting the canonical Session file.

Approach

Add PiWebAdapter.unarchiveSession using the existing atomic archive metadata transaction and expose POST /api/sessions/unarchive. The operation validates the canonical Session through the same adapter boundary and emits a session_unarchived event.

Validation

• npx tsc --noEmit
• node --test --experimental-strip-types tests/web/web-host.test.ts tests/web/pi-adapter.test.ts (32 passed)
• git diff --check

Impact

• User-visible behavior: restores an archived Session through the API.
• Model-visible context/tools: none.
• Runtime/lifecycle: no Session replay or side effects.
• Persisted config/data: updates existing archived-sessions.json atomically.
• Compatibility/risk: archive view/UI action remains a follow-up; existing archive behavior is unchanged.