fix: detect embedded agent runtimes

[SaladDay]

Summary

• detect Claude Code, OpenCode Desktop, Hermes, and Pi from their exact runtime markers
• preserve executable PATH detection and the existing Codex runtime detection
• reject broad, generic, and near-match environment markers to reduce false positives

Scope

This PR only changes agent availability detection and its tests. It does not change installation, configuration, command behavior, dependencies, versions, or Grok detection.

Validation

• targeted availability tests: 10 passed
• typecheck: passed
• lint: passed with one pre-existing warning in test/sdk/speech.test.ts
• build: passed
• full suite: 508 passed; one unrelated installer process-tree timeout test failed before writing its helper PID
• two independent blind reviews plus one final blind review: no findings

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