refactor: TypeScript config improvements and code quality fixes

[roottool]

Description

Tooling and TypeScript configuration improvements identified during bundler experiments on this branch.
No changes to runtime behavior or public API.

Type of Change

• Refactoring (non-breaking change improving code structure)
• 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

Changes

TypeScript Config

• isolatedDeclarations: true enabled — prep for Oxc-based .d.ts generation
• isolatedDeclarations: false override added to tsconfig.examples.json (examples don't emit .d.ts)
• removeComments removed — redundant since minifiers handle it; JSDoc now preserved in .d.ts

Package Config

• sideEffects: false added for reliable tree-shaking

Code Quality

• FORBIDDEN_KEYS: changed to ReadonlySet<string> — stronger immutability, prevents .add()/.delete()/.clear() at the type level
• src/index.ts: removed internal JSDoc comment that leaked into .d.ts as a meaningless note to consumers
• src/index.ts: moved import to top per convention; simplified re-export from from "..." form

Testing

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

---

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (ec01447) to head (174a7ef).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #63   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           23        23           
  Branches         5         5           
=========================================
  Hits            23        23           

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