feat(queue): add opt-in error callback and standardize README

[usernane]

Summary

Adds an opt-in error callback so applications can observe job-processing failures, and standardizes the README.

Motivation

• feat(queue): expose job-processing exceptions (swallowed in Queue::process()) #9: Queue::process() caught every Throwable from Job::handle() and discarded everything but the message string. A globally registered error handler never saw queue-job failures, intermediate (pre-max-attempts) failures were recorded nowhere, and consumers had no failure hook. This makes background work a blind spot for centralized error logging.
• docs: standardize README.md structure #6: The README was missing several sections required by the ecosystem standard.

Closes #9 and #6.

Changes

#9 — expose job-processing exceptions

• Queue::setOnError(?callable $cb) / getOnError() — an opt-in callback invoked for every caught throwable, with signature fn(?Job $job, \Throwable $e, int $attempts, bool $willRetry): void. It fires for both intermediate retries ($willRetry = true) and terminal failures ($willRetry = false).
• Failed jobs now record richer context: getFailReason() includes the exception class in addition to the message.
• Invalid (non-Job) payloads are reported to the callback as terminal failures with a null job (and still marked failed).
• QueueFacade::setOnError() passthrough.
• Fully additive/opt-in: with no callback registered, the catch → retry / mark-failed behavior is unchanged.

#6 — standardize README

• Added Table of Contents and Testing / Examples / Contributing / Support / Changelog sections.
• Documented the new setOnError (an "Observing Failures" section) and added it to the API table.
• Fixed an inaccurate example (getFailed() returns QueuedJob[], so retry($failed[0]->getId()), not array access).

How to Test / Verify

• composer test → 38 tests pass (was 33; +5 new covering: onError on terminal failure, intermediate retry, invalid payload, chainability/getter, facade passthrough). Two existing tests updated for the enriched failReason format.
• All 6 README PHP code blocks verified with php -l.
• composer fix-cs clean.

Breaking Changes and Migration Steps

None. The callback is opt-in. The only observable change is that a failed job's getFailReason() now includes the exception class prefix (e.g. RuntimeException: ...) in addition to the message.

Checklist

• I reviewed my own diff before requesting review
• My commits follow Conventional Commits
• I added/updated tests (or explained why not)
• I updated docs (if needed) Docs Repo — README updated
• I ran lint/cs-fixer (if applicable) (composer fix-cs)
• I considered backward compatibility
• I considered security

Related issues

Closes #9, Closes #6

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.72%. Comparing base (3eb9bf2) to head (9a203ca).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main      #10      +/-   ##
============================================
+ Coverage     96.03%   97.72%   +1.69%     
- Complexity       78       84       +6     
============================================
  Files             4        4              
  Lines           252      264      +12     
============================================
+ Hits            242      258      +16     
+ Misses           10        6       -4     

Flag	Coverage Δ
php-8.1	97.72% <100.00%> (+1.69%)	⬆️
php-8.2	97.72% <100.00%> (+1.69%)	⬆️
php-8.3	97.72% <100.00%> (+1.69%)	⬆️
php-8.4	97.72% <100.00%> (+1.69%)	⬆️
php-8.5	97.72% <100.00%> (+1.69%)	⬆️

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud