docs(scheduler): expand PRD with Scheduler Extension and add sectools…

[hyp3rd]

… dependency

• Update PRD title to “Retrier Hardening + Scheduler Extension”.
• Document new lightweight scheduler for recurring HTTP requests:
  • Interval scheduling with StartAt/EndAt/MaxRuns and concurrency-safe execution.
  • Reuse existing retrier for retryable codes/errors; stop on success or non-retryable statuses.
  • Optional callback URL to post execution results (bounded response body; skipped when empty).
  • Default URL validation via sectools (HTTPS only, no userinfo/private/localhost), with override/disable knob.
• Clarify scope: no persistent storage, cron syntax, or non-HTTP protocols.
• Add detailed Requirements, Acceptance Criteria, Test Plan, Decisions, and Status sections.
• Update Dependencies to include github.com/hyp3rd/sectools.

Refs: PRD.md

[Copilot]

Pull request overview

This PR expands the PRD to document the Scheduler Extension, a new lightweight scheduler for recurring HTTP requests with retry support and optional callbacks, and adds the github.com/hyp3rd/sectools dependency.

Changes:

• Updated PRD title to "Retrier Hardening + Scheduler Extension"
• Added comprehensive documentation for the Scheduler Extension including goals, requirements, acceptance criteria, test plan, and decisions
• Added github.com/hyp3rd/sectools to the dependencies list

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[Copilot]

The phrase "no behavioral changes" is misleading in the Status section for the Scheduler Extension. Since the scheduler is new functionality, it would be more accurate to say something like "Tests and race checks pass; scheduler is tested with TLS endpoints and validation coverage" or "Implementation complete with comprehensive test coverage and race-free operation." The current wording suggests no changes occurred, which contradicts the fact that this is a new feature.

Suggested change

	- Tests and race checks are reported green with no behavioral changes.
	- Tests and race checks are reported green; scheduler implementation is complete with TLS endpoint coverage, validation tests, and race-free operation.