feat: configure slow-run threshold

[Rayan-and-beyond]

Summary

• add --slow-seconds to agentrace check while preserving the 900-second default
• thread the configured threshold into analyse only for the runaway check
• cover raised and lowered thresholds and document the flag

Closes #6

Validation

• PYTHONPATH=. uv run --extra dev pytest -q — 56 passed
• uv run --extra dev ruff check . — passed
• focused slow-run tests — 2 passed
• mutation check: ignoring the configured threshold makes the focused configurable-threshold test fail
• git diff --check — passed

[github-actions]

And it's in. Thanks @Rayan-and-beyond.

Genuinely helpful change for Agentrace.

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Happy to see more from you whenever you have the time.