Update README to indicate package is archived

[roottool]

Added caution note about package maintenance status.

Description

Type of Change

• Bugfix (non-breaking change fixing an issue)
• Security fix (non-breaking change addressing a security issue)
• Documentation (changes to documentation only)
• Refactoring (non-breaking change improving code structure)
• Tests (adding or updating tests)
• Tooling / CI (changes to build tools, CI configuration)

Boundary Checklist

• This is a non-implementation change (Documentation, Tooling, or CI only)
• No interpretation — Keys treated as opaque strings, no structural inference
• No silent behavior — No merging, overwriting, or implicit resolution
• Boundary respected — No validation, coercion, or business logic

📘 AGENTS.md

Security & API Stability

• No security impact (forbidden keys, prototype safety unaffected)
• No changes to public API (parse, ParseResult, ParseIssue, IssueCode)
• No breaking changes; or justified with version bump rationale
• Compatible with current v0.x versioning policy

Testing

• bun run check:type:source passes
• bun run test passes
• bun run build passes