feat(errors): implement ErrorInstance.addNote()

[martyy-code]

Closes #29.

Mirrors Python 3.11 PEP 678 (BaseException.add_note()). The method was documented but never implemented; consumers following the JSDoc examples got a TypeScript error. The notes: string[] storage was already wired up; only the method was missing.

Why

• The discrepancy between docs and implementation is a real bug. Anyone copying the JSDoc example from src/raise/index.ts:34-38 gets a TypeScript error.
• The feature is already designed and the storage property is already implemented (src/error/error.ts:107, tested at tests/error.test.ts:62-63). The remaining work is small and additive.
• Useful feature: notes let you enrich errors with runtime context without changing the API. Python 3.11 PEP 678 ships this exact pattern; porting it is consistent with the library's Python inspiration.

What changed

• packages/errors/src/error/types.ts: declare addNote(note: string): ErrorInstance<TFields> on ErrorInstance. Remove the stale // TODO: Implement .addNote() method (Task XX) comment and the // Note: .addNote() is implemented in a separate task. notice.
• packages/errors/src/error/error.ts: implement addNote in the factory closure. Pushes to notes and returns this for chaining.
• packages/errors/tests/error.test.ts: cover single note, chained notes, preservation through .from(), and isolation between sibling instances.
• .changeset/add-addnote-method.md: minor bump for the new API.

Code sample

Before:

const err = AppError();
// Property 'addNote' does not exist on type 'ErrorInstance<TFields>'.

After:

const err = AppError()
  .addNote('Attempt 1 failed')
  .addNote('Retrying...');
// err.notes === ['Attempt 1 failed', 'Retrying...']

Verified locally

• pnpm test --run — 82/82 tests pass
• pnpm lint
• pnpm type-check
• pnpm build

Risk

Low. .addNote() is purely additive; existing code is unaffected. The only contract change is the return-type inference on the new method, and it is fully typed (ErrorInstance<TFields>).

Note on CI tooling

This PR was committed with --no-verify. The pre-commit hook runs pnpm exec lint-staged from the monorepo root, but eslint --fix is invoked without cwd=packages/errors, so it cannot find the package-local eslint.config.js. This is a pre-existing repo issue, not caused by this PR. A follow-up issue to fix the husky / lint-staged setup for the monorepo would be welcome.