[codex] Support repeated exec usings

[splusk2006]

Summary

• Allow unity-cli exec --usings to be specified multiple times.
• Normalize comma-separated and repeated --usings values into one de-duplicated array before sending params to Unity.
• Canonicalize discovered project paths before cwd matching so macOS symlink aliases such as /var and /private/var resolve to the same Unity project, while preserving Windows case-insensitive matching.
• Update CLI help, README docs, and the connector parameter description.

Root Cause

buildParams stored subcommand flags in map[string]string, so repeated flags overwrote earlier values. exec --usings A --usings B only sent the final namespace to the Unity connector even though the connector already accepts an array.

While validating with the repo Go version, an existing internal/client test also exposed a project path normalization gap on macOS: cwd could be reported through /var/... while Unity instance data used /private/var/.... That made DiscoverInstance miss the active cwd project and fall through to the multi-instance error.

Validation

• Downloaded and used official go1.24.3 darwin/arm64 locally from go.dev/dl to match go.mod.
• /tmp/unity-cli-go-1.24.3/go/bin/gofmt -w cmd/root.go cmd/root_test.go internal/client/client.go
• git diff --check
• /tmp/unity-cli-go-1.24.3/go/bin/go test ./internal/client -run TestDiscoverInstance_UsesCwdProjectMatch -count=1 -v
• /tmp/unity-cli-go-1.24.3/go/bin/go clean -testcache && /tmp/unity-cli-go-1.24.3/go/bin/go test ./...