Add installer script and CLI dispatch test coverage - #12

Summary

Adds a curl-to-bash installer for the latest release and expands test coverage across the CLI dispatch, note form, and frontmatter paths. Ships the v0.4.1 version bump.

What's new

• Installer script - install.sh detects OS and architecture, resolves the latest GitHub release, and installs park to /usr/local/bin, with PARK_VERSION and positional version overrides.
• CLI dispatch tests - cmd/park extracts newCommand() from Main() so tests drive the full command tree without touching os.Args or process exit.
• Note and form tests - new coverage for note frontmatter add/parse, file ingestion, and the Bubble Tea form model.

Behavior changes / guardrails

• The installer falls back to a pinned version when the latest-release redirect can't be resolved, and exits non-zero on download, extraction, or move failures, pointing at the releases page.
• Installation targets /usr/local/bin and requires sudo.

Testing

• Added coverage across cmd/park (dispatch and exit codes), internal/note (frontmatter and ingestion), and internal/model (form model).
• make check runs fmt, vet, lint, and tests; all green.