Fix recursive note imports and large history record scans

[fitchmultz]

Summary

• Prevent legacy notes migration from importing the shared destination back through a directory symlink, while continuing to import other linked files.
• Accumulate unfinished JSONL record chunks and join them once, instead of copying and rescanning the growing record for every chunk.

Verification

• Added a regression covering the reproduced worktree symlink layout: it fails with ENAMETOOLONG before the fix and passes afterward, including notes list/read/write and importing ordinary linked files.
• Four focused migration/history tests pass, including Unicode separators, multi-chunk records, an unterminated final record, and separate Git-directory layouts.
• npm ci --ignore-scripts --no-audit and npm run check pass.
• Runtime tests and the benchmark use the installed Pi fork SDK (e53367c, 0.87.1), through a temporary module resolver; CI qualifies the repository-pinned fork and official-host refusal.
• Same bounded-setup 64 MiB archived image-data record: history search took 3,493 ms before / 64 ms after; peak process RSS was 963 MiB before / 484 MiB after. These are local measurements, not timing assertions added to the suite.

No new dependencies or settings.