[P2][OCR] Cache CLI discovery search directories per detection pass

[Ryder-Sun]

Source: OCR_ANALYSIS_REPORT.md, OpenCode Review report generated on 2026-08-12.

Finding: M-10. desktop/src/agents/cli/cli-discovery.cjs rebuilds search directories repeatedly across Agent kinds and probes, including redundant filesystem reads. This can slow startup Agent detection on machines with many local tool paths.

Expected fix:

• Cache computed search directories for a detection pass or stable options signature.
• Preserve platform-specific Finder, PATH, npm, and Windows shim behavior.
• Add tests proving repeated probes do not recompute directory scans unnecessarily.