feat(drive): add --debug NDJSON sync log to drive watch

[yurenju]

Summary

• Add --debug flag (and WSPC_DRIVE_DEBUG=1) to wspc drive watch: appends NDJSON debug events to <folder>/.wspc-drive/debug.log with 10MB rotation to debug.log.old; write failures never break sync.
• Events: fs_event (raw pre-debounce), realtime_event, sync_start (trigger: local/remote/initial/retry), sync_end (duration + summary), sync_phases (scan/manifest/process ms), decision (per actionable path with base/local/remote comparison fields; unchanged paths skipped), conflict (type/strategy), retry.
• Sync upload/delete requests now send x-wspc-client: drive-sync so the server can attribute drive_file_operation events to the sync loop vs generic API traffic.

Spec: docs/superpowers/specs/2026-07-13-sync-debug-log-design.md in wspc-drive. Companion PRs in wspc-drive (dashboard toggle) and wspc (PostHog events).

Test plan

• New test/handwritten/drive/debug-log.test.ts: writer NDJSON schema, rotation, error swallowing, watch wiring (fs_event/sync_start/sync_end/retry), sync decision/conflict/phases events, no file when debug off.
• New api test: x-wspc-client header on upload/delete.
• Full suite green (config-lock concurrent-writer test is a known Windows flake, passes standalone).

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