Make server JSON decoding more idiomatic Effect

[cursor]

What Changed

• Updated trace diagnostics NDJSON aggregation to decode span lines through Schema.fromJsonString via the shared JSON helper instead of raw JSON.parse and manual unknown record traversal.
• Tracked trace span bounds internally with Option instead of nullable state.
• Added Effect-returning GitHub PR JSON decoder variants and wired GitHubCli to map schema decode failures directly to existing tagged CLI errors.
• Added focused regression coverage for schema-invalid trace lines and invalid GitHub PR JSON output.

Why

This keeps two server JSON parsing paths aligned with the repo's idiomatic Effect patterns: schema-driven parsing, structured tagged errors, Effect-returning decoder APIs, and explicit Option state where values may be absent.

Checklist

• This PR is small and focused
• I explained what changed and why
• UI changes not applicable

Validation:

• vp check
• vp run typecheck
• pnpm exec vp test run src/diagnostics/TraceDiagnostics.test.ts src/sourceControl/GitHubCli.test.ts (from apps/server)

  

Note

Make JSON decoding in server trace and GitHub PR modules use Effect idioms

• Introduces TraceRecordLineSchema in TraceDiagnostics.ts to validate trace line structure with a typed schema; lines failing schema validation now increment parseErrorCount alongside JSON parse failures.
• Adds Effect-returning decoders (decodeGitHubPullRequestJsonEffect, decodeGitHubPullRequestListJsonEffect) in gitHubPullRequests.ts and updates GitHubCli.ts to surface decode failures as tagged errors (GitHubPullRequestDecodeError) in the Effect error channel.
• Converts internal helpers (toStringValue, unixNanoToDateTime, readExitTag, readExitCause) from nullable returns to Option<T> types throughout TraceDiagnostics.ts.
• Behavioral Change: parseErrorCount in trace diagnostics now counts schema validation failures in addition to raw JSON parse errors.

^{Macroscope summarized 55038ce.}