Add public schema package and park schema command

[polymorcodeus]

Summary

Adds a public schema package that owns the canonical frontmatter contract and exposes it through park schema and park schema --json. This lets downstream tooling import the contract instead of re-deriving field names, categories, and the write template.

What's new

• Public schema package - canonical Frontmatter struct, category enum (inbox, projects, areas, archive), SchemaVersion, date format, and write template; stdlib only so other tools can import it.
• internal/note uses the contract - note.Metadata is now an alias to schema.Frontmatter; note.Write and note.Today read the template and date format from schema.
• Config and TUI reference the enum - default categories use schema.Category* constants and internal/model/assist.go parses dates with schema.DateFormat.
• park schema command - human-readable and JSON output surfaces the contract for scripts and downstream consumers.

Usage

Command	Purpose
park schema	print the human-readable frontmatter contract
park schema --json	print the machine-readable contract

Behavior changes / guardrails

• schema is a pure leaf package with no internal or Bubble Tea dependencies, so importing it from other modules is safe.
• park schema runs through the normal Before hook but falls back to default config when no config file exists, so it works on a fresh install.
• schema_version starts at 1 and increments only on breaking contract changes.

Testing

• Unit tests in schema/schema_test.go cover categories, Frontmatter completeness, Render, Describe, and JSON round-tripping.
• Existing tests across internal/note, internal/config, internal/model, and internal/store continue to pass against the aliased Metadata.
• make check runs fmt, vet, lint, and tests; all green.