feat(retrier): per-attempt error tracking, early stop, and init guards

[hyp3rd]

• Track errors per attempt (populate Errors.Attempts); remove shared r.err usage. • Return early when a non-temporary error is encountered (when a temporary-error list/registry is provided). • Wrap timeouts with ErrTimeoutReached and include attempt context. • Add ensureInitialized/ensureContext; default registry, logger, timer pool; support manual initialization. • Tests: cover early-stop behavior, timeouts, and manual init (e.g., TestDo_NonTemporaryStopsEarly, TestDo_ManualRetrierInitialization). • Docs: add PRD for Retrier Hardening; update README and examples (temporary error registry usage).

BREAKING CHANGE: Errors.Retries renamed to Errors.Attempts. Update usages and expectations: • Use Errors.Attempts for attempt traces.
• Register temporary errors via retrier.Registry.RegisterTemporaryError. • Expect timeout errors as ErrTimeoutReached and max-retries errors wrapped by ErrMaxRetriesReached.

[Copilot]

Pull request overview

This pull request hardens the go-again retrier implementation by fixing concurrency issues, improving error handling, and adding support for manually constructed Retrier instances. The PR introduces per-attempt error tracking, early stopping on non-temporary errors, and proper timeout error wrapping.

Changes:

• Removed shared mutable state (r.err field) to enable safe concurrent calls to Do
• Added initialization guards (ensureInitialized, ensureContext) to support manual Retrier construction without NewRetrier
• Implemented early return when non-temporary errors are encountered with a temporary error list provided
• Wrapped timeout errors with ErrTimeoutReached for errors.Is compatibility

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
retrier.go	Core implementation changes: removed shared error state, added per-call error tracking, implemented initialization guards, improved timeout error wrapping
tests/retrier_test.go	Added tests for early-stop behavior, timeout error detection, and manual Retrier initialization
README.md	Updated API documentation to reflect Errors.Attempts field rename and corrected registry usage examples
PRD.md	Added Product Requirements Document describing the hardening goals and implementation approach

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Copilot]

The "Rollout" section contains duplicated and incomplete text fragments. The section should be properly completed or removed if not needed. The text "arly on non-temporary errors when a list is provided" appears to be a copy-paste error from the "Proposed Changes" section.

Suggested change

	arly on non-temporary errors when a list is provided.
	arly on non-temporary errors when a list is provided.
	- Update README snippets and API notes to match current code.
	- Release changes as a minor version (no breaking API changes).
	- Coordinate with dependent services to update the `go-again` dependency and run regression tests.
	- Monitor retry behavior, error rates, and timeout frequency after rollout.
	- Be prepared to roll back to the previous version if regressions in retry or timeout handling are observed.