fix(cli): centralize provider executable resolution

[cododel]

Summary

• Show complete failed-session diagnostics on hover or keyboard focus, with a copy action.
• Resolve provider CLI executables once at startup and reuse the same absolute launcher across terminal, limits, and agent-browser flows.
• Add packaged macOS smoke coverage for minimal-PATH CLI discovery and structured missing-CLI failures.

Verification

• npm test (420/420)
• npm run typecheck
• npm run build
• npm run package:mac
• npm run smoke:cli-resolution:mac
• npm run audit:secrets

Closes #20