test(retention): verify MaxEntries pruning under load

[hyp3rd]

Add a retention stress test ensuring the task registry is pruned down to MaxEntries after tasks reach terminal states. Update README to clarify retention semantics (only terminal tasks are evicted), best-effort cleanup timing, and the default CleanupInterval behavior.

[Copilot]

Pull request overview

This PR adds a stress test to verify that the task registry enforces MaxEntries under higher load and clarifies retention semantics and behavior in the README.

Changes:

• Add TestTaskManager_RetentionMaxEntriesUnderLoad to ensure the registry is pruned down to MaxEntries after tasks reach terminal states, even under higher throughput and larger max task limits.
• Introduce load-specific retention constants in the worker tests (retentionLoadTasks, retentionLoadMaxEntries, retentionLoadMaxTasks).
• Document retention semantics in the README, including which tasks are eligible for eviction, cleanup timing, and the default CleanupInterval behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
tests/worker_test.go	Adds a high-load retention test and supporting constants to validate MaxEntries pruning and terminal-state-only retention logic.
README.md	Expands documentation of retention semantics, cleanup behavior, and default CleanupInterval computation.

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[Copilot]

The for range retentionLoadTasks loop is invalid because retentionLoadTasks is an int constant, so this code will not compile. Use an index-based loop (or create the slice with a fixed length and index into it) to append retentionLoadTasks items to the tasks slice.

Suggested change

	for range retentionLoadTasks {
	for i := 0; i < retentionLoadTasks; i++ {