docs: document error-class selection guidelines

[tomas-zijdemans]

This writes down the error-class doctrine that #5340 established but CONTRIBUTING.md never captured: which built-in to throw, when to subclass one, and when a custom class is justified. AGENTS.md gets the condensed version, next to the message-style rules it already mirrors.

Nothing here is invented. It comes from a throw new survey of main (tests excluded):

• TypeError for wrong argument type or shape: http 55, cbor 21, data-structures 15
• RangeError for out-of-range values: cbor 33, async 15, encoding 12
• SyntaxError for unparsable textual input: http 59, toml 21, csv 10, jsonc 10, ini 5. Established by the suggestion: replace YamlError with native errors #5340 campaign (BREAKING(yaml): replace YamlError with SyntaxError in parse() #5446, BREAKING(yaml): replace YamlError with TypeError in stringify() #5452, BREAKING(csv): throw TypeError in stringify() instead of StringifyError #5347, BREAKING(toml): throw SyntaxError in parse() instead of TOMLParseError #5346) to align parse() functions with JSON.parse()
• Built-in subclasses carrying position info: XmlSyntaxError (BREAKING(xml): clean up public API #7233), YamlSyntaxError (feat(yaml): introduce YamlSyntaxError with structured position info #7235)
• Custom classes only for domain conditions callers catch: RetryError, ChannelClosedError, CircuitBreakerOpenError, MockError, TimeError, HttpError

Plain Error (datetime 19, fs 18) and EvalError (msgpack 27) predate the campaign, so the text calls them legacy instead of pretending the tree is uniform.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.03%. Comparing base (b847d39) to head (1a4b3ba).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7279      +/-   ##
==========================================
- Coverage   95.03%   95.03%   -0.01%     
==========================================
  Files         618      618              
  Lines       51598    51850     +252     
  Branches     9345     9394      +49     
==========================================
+ Hits        49036    49275     +239     
- Misses       2021     2030       +9     
- Partials      541      545       +4     

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[bartlomieju]

A bit wary of the duplication - maybe AGENTS.md could reference the CONTRIBUTING.md?

[tomas-zijdemans]

Fair, done. Now a reference. Fwiw the Error Message Style section above duplicates CONTRIBUTING.md the same way, so I can convert that one too in a follow-up if you want AGENTS.md to be pointers rather than copies.

[bartlomieju]

That would be great, thanks!