feat(conflict): introduce a three-way sync base and preserve both sides

[ClaudiaFang]

Priority

P1 — conflict safety

Problem

Current conflict detection can identify that both local and remote changed, but it does not maintain a complete three-way model:

base (last synced)
local
remote

Without the base version, the plugin cannot safely distinguish overlapping from non-overlapping edits or attempt an automatic merge.

Plan

• Persist the last-synced base blob identity for each tracked file.
• Retrieve base, local, and remote contents when a conflict is detected.
• Introduce an explicit conflict model with path, base, local, remote, and resolution state.
• Attempt automatic merge for non-overlapping text changes.
• Preserve both sides when automatic merge is unsafe.
• Define conflict artifacts or snapshots without overwriting the original silently.
• Integrate the model with batch conflict handling in feat: support batch pull when multiple files have conflicts #95.
• Define behavior for binary files, deleted files, moves, and unavailable base blobs.

Acceptance criteria

• No conflict resolution path silently discards local or remote content.
• Non-overlapping text edits can be merged deterministically.
• Overlapping changes produce an explicit unresolved conflict.
• Base metadata survives reopen/restart and has migration coverage.
• Unit tests cover text merge, overlapping edits, delete/modify, move/modify, binary files, and missing base blobs.