Add Parallel Decisions API to the existing Parallel integration #1285

What problem does this solve?

Treg already exposes Parallel's search, extraction, and entity-search APIs, but its Parallel catalog does not expose the Decisions API. Users who want web-grounded classification, including several named questions sharing one context, currently need a separate Parallel integration.

Proposed solution

Add a direct Decisions tool to the existing Parallel catalog, using POST https://api.parallel.ai/v1/decisions and the existing x-api-key credential integration.

The tool should:

• Accept shared state and one or more named questions, preserving the documented noul, choice, and score question types and optional fields.
• Relay vendor responses with decision values, confidence/probability fields, usage, and citations intact.
• Follow existing treg conventions for invalid requests, authentication errors, rate limits, and upstream failures.
• Prefer team-owned Parallel credentials and leave the team's treg balance untouched for those calls.
• Document verified vendor pricing and keep platform-funded access gated until platform-key billing and settlement behavior are verified live.
• Include a sanitized fixture, focused verification, and a minimal CLI example with a ledger distinguishing mocked tests from live verification.

A proposed implementation is already open in #1283. This issue records the use case and scope for maintainer discussion. That PR's verification uses mocked upstream responses; live Decisions calls and billing have not been verified, and platform-funded access remains gated.

Official references:

• Decisions announcement and request example
• Decisions quickstart

Alternatives considered

Calling Parallel directly works but requires users to maintain a separate integration and credential flow. Extending the existing catalog lets them use treg's standard discovery and call interfaces. Automated provider routing and additional Parallel APIs can be addressed separately.