Add sync watcher and conflict resolution spec

[Intina47]

Motivation

• Enable safe sync by adding a local filesystem watcher to detect changes to note files for sync operations.
• Define a deterministic conflict resolution policy (last-write-wins with preserved conflict copies) so concurrent edits are recoverable.

Description

• Add watcher.go which implements a polling Watcher with NewWatcher, Poll, Start, and snapshotDir, emitting FileEvent values for create, modify, and delete with deterministic ordering using sort.Slice.
• Add watcher_test.go to validate detection of create/modify/delete transitions using Poll in a temporary directory.
• Add sync_conflicts.go with conflictCopyPath that produces deterministic conflict filenames of the form <original>.conflict-<UTC timestamp>-<actor id> (actor defaults to unknown).
• Add sync_conflicts_test.go to assert conflict filename formatting and default actor behavior, and add docs/sync-conflicts.md documenting conflict detection and the last-write-wins + conflict-copy policy.

Testing

• Ran the full test suite with go test ./... and all tests passed.

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Codex Task

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Sanitize actorID to prevent path traversal

Because actorID is interpolated directly into filename and then passed to filepath.Join, any actor ID containing path separators (e.g., ../ or ..\) will be treated as path components and can escape the note directory after path cleaning. This allows a remote sync client identifier to place conflict copies outside the intended directory (or overwrite unrelated files) instead of always writing alongside the original. Consider normalizing actorID to a safe character set (e.g., replacing path separators with _) before building the filename.

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