feat(web): expose Pi thinking state

[seekskyworld]

Problem

Adds the reasoning-state inspection slice of #348. The Web workbench could not show the active Pi thinking level or model-supported levels.

Value

Users can inspect reasoning configuration from the browser while Pi remains authoritative.

Approach

Expose optional runtime getThinkingState() backed by Pi AgentSession and an authenticated GET /api/thinking endpoint. Existing controllers remain compatible; no credentials or configuration writes are introduced.

Validation

• npx tsc --noEmit
• git diff --check

Impact

• User-visible behavior: adds bounded reasoning-state inspection.
• Model-visible context/tools: none.
• Runtime/lifecycle: read-only Pi state projection.
• Persisted config/data: none.
• Compatibility/risk: additive optional seam; setting changes remain future work.