misc(changelog): correct --before → --since in fragment for #260

[decko]

Summary

Ticket #311 identified that the towncrier changelog fragment for PR #260 incorrectly referenced --before DATE instead of --since DATE for the raki cohort command.

The fix was already applied in commit 2ac5935 ("chore: add towncrier fragment for #260"), merged via PR #310, and released in v0.14.0. CHANGELOG.md line 15 now correctly reads --since DATE.

This PR carries no source-code changes — only the towncrier fragment for the ticket itself.

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Acceptance Criteria

• CHANGELOG.md references --since DATE (not --before DATE) for the raki cohort command (feat: raki cohort command — date-based split and diff within a single report #260)
• All tests pass (1845 passed, 4 skipped, 4 deselected)
• No regressions in linting or formatting

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Review Results

Specialist	Verdict	Findings
Implementation	✅ PASS	Fix already applied in v0.14.0; no further changes required
Verification	✅ PASS	CHANGELOG.md correct; source code has zero --before references
Review	✅ PASS	0 findings

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Refs #311

Assisted-by: Claude Opus 4.6 (1M context) noreply@anthropic.com
Assigned-by: decko