fix(#17): dedupe rsync --exclude emissions after **/ normalization

[eloualiche]

Summary

• Factor the two `--exclude=` emission loops in `Syncer.BuildCommand()` into a new `appendExcludes` helper that dedupes on the normalized (post-`strings.TrimPrefix("**/")`) pattern.
• Pre-existing quirk: `ignore = [".venv/", "**/.venv/"]` (a common defensive double-form) produced two identical `--exclude=.venv/` args. rsync silently ignored the duplicate so there is no correctness bug, but `--verbose` output was noisy.
• TDD: two new tests that fail on `main` and pass after the fix, covering both the `include`-set and no-`include` branches of `BuildCommand`.

Closes #17.

Test plan

• New `TestBuildCommand_DedupExcludes` and `TestBuildCommand_DedupExcludes_WithInclude` pass
• Existing `TestBuildCommand_ExcludePatterns`, `TestBuildCommand_IncludePatterns`, `TestBuildCommand_ExcludesBeforeIncludes` still green
• `go test ./...` green locally; CI to confirm