Add park list for Grouped Note Browsing and JSON Export - #13

Summary

Adds park list, a read-only command that lists parked notes grouped by category with frontmatter-style output and a versioned --json envelope for scripted consumers. Introduces a per-category excluded flag so a category like archive stays out of the default listing until explicitly requested.

What's new

• Listing command - park list (alias ls) renders one banner per category, then one block per note showing filename, category, created, source, and synopsis, ordered oldest-modified first.
• Filtering - -c, --category restricts the listing to named categories (repeatable) and overrides exclusion; --all includes categories excluded by config.
• Machine output - --json emits a flat envelope of {schema_version, items} where each item carries filename, path, category, created, source, synopsis, and a UTC RFC 3339 modified; schema_version tracks the contract from park schema.
• Config exclusion - new per-category excluded boolean; the built-in IPAA config ships with archive excluded.

Usage

Behavior changes / guardrails

• Explicit --category overrides the default exclusion and --all; --all only widens the default selection.
• Unknown --category values exit 2 with the valid list, matching park new and park reclassify.
• --json always emits items as an array, never null, and reports the folder-derived category so a stale frontmatter category never disagrees with the banner.
• excluded is encoded with omitempty, so non-excluded categories are unchanged in park config output.
• Listing is read-only; park assist and the reclassify triage flow are unaffected.

Testing

• Store tests cover excluded-vs---all selection, explicit-category override, unknown category, empty-group omission, plain formatting, and the JSON envelope (version, fields, UTC modified).
• CLI dispatch tests cover list, alias ls, --all, --category override, unknown-category exit 2, --json parsing, and --json --category filtering.
• Config tests cover the archive default and the excluded round-trip through Dump.
• make check runs fmt, vet, lint, and tests; all green.