fix(cli): add log rotation for fern docs dev debug logs

[thesandlord]

Description

Closes FER-9568

fern docs dev writes debug logs to ~/.fern/logs/ but never cleans them up, causing unbounded disk usage over time. This adds automatic log rotation with a 100 MB directory size cap.

Changes Made

• Added enforceLogSizeLimit() private method in DebugLogger that runs fire-and-forget at the end of initialize()
• Scans the logs directory, sorts files by mtime (oldest first), and deletes until total size is ≤ 100 MB
• Skips the current session's log file to avoid deleting active logs
• Added optional consoleLogger callback parameter to DebugLogger.initialize() so rotation status is logged to the console (debug when under cap, info when rotating) — wired to context.logger in runAppPreviewServer.ts
• All errors are silently caught to avoid breaking the main fern docs dev flow
• Added versions.yml entry (4.63.2, fix)

Review Checklist

• file.fullPath === this.logFilePath comparison: this.logFilePath is an AbsoluteFilePath (branded string), while file.fullPath is built with path.join() (plain string). Works at runtime since AbsoluteFilePath is string & { __AbsoluteFilePath: void }, but verify paths are normalized consistently.
• Cleanup runs fire-and-forget (void) — does not block startup
• Cleanup only runs at startup — a single very long session could still exceed 100 MB. Acceptable tradeoff.

Testing

• Unit tests added/updated — not added; cleanup logic is defensive (all errors caught) and straightforward
• Lint passes (biome check)
• Verified console output shows rotation status with --log-level=debug

Link to Devin session: https://app.devin.ai/sessions/0212b2e6da7543248d179fb0d4f0450a
Requested by: @thesandlord

[devin-ai-integration]

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[thesandlord]

dont await, run in bg

[devin-ai-integration]

Good call — changed to void this.enforceLogSizeLimit(logsDir) so it runs fire-and-forget without blocking startup.

[thesandlord]

add a debug log here "rotating logs" or something like that

[devin-ai-integration]

Added a log_rotation debug entry that logs total size, file count, and the cap before deletion starts.