feat(tracking): identify CLI and detect AI agent harness

[angeloashmore]

Resolves:

Description

Adds a detectAgent helper that identifies the AI coding agent (Claude Code, Codex, Cursor, Cline, Copilot, etc.) invoking the CLI based on environment variables. Groundwork for harness-aware tracking.

Checklist

• A comprehensive Linear ticket, providing sufficient context and details to facilitate the review of the PR, is linked to the PR.
• If my changes require tests, I added them.
• If my changes affect backward compatibility, it has been discussed.
• If my changes require an update to the CONTRIBUTING.md guide, I updated it.

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Note

Low Risk
Low risk: adds a new standalone helper that only reads environment variables (and optionally checks a Linux file path) without changing existing execution paths.

Overview
Introduces src/lib/ai.ts with a new async detectAgent() helper that identifies the active AI agent by checking a prioritized set of environment variables (Claude Code/Cowork, Codex, Cursor, Cline, Copilot, etc.).

On Linux it additionally probes for a Devin marker at file:///opt/.devin, and it returns unknown-sandbox when IS_SANDBOX=yes.

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