Add change-only notification option and change summaries

[happyloa]

Motivation

• Reduce noisy alerts by sending notifications only when risks are newly added or changed compared to the previous scan.
• Provide a concise change summary in notification payloads so recipients can quickly see what changed.
• Reuse existing history storage to compute diffs rather than sending full risk lists when not needed.
• Ship the feature as a user-configurable setting and bump the plugin version to reflect the change.

Description

• Add RiskRepository::diffWithLatest and supporting helpers (normalizeRiskArray, buildRiskSignature, signature map builders, and comparison normalization) to compute added/changed risks from stored history.
• Inject RiskRepository into Notifier and optionally replace the full risk list with added + changed when notify_changes_only is enabled, and add a human-readable change summary to email, Slack, Teams, Discord, and webhook outputs.
• Add notify_changes_only to SettingsRepository with sanitization and persistence, and add a checkbox to the admin settings UI in templates/admin-page.php (using existing form/nonce flows and checked/esc_attr/esc_html).
• Wire up the constructor call in site-add-on-watchdog.php, bump Version::NUMBER and top-level plugin Version to 1.8.0, and add a short changelog entry in readme.txt.

Testing

• No automated tests were executed as part of this change.
• The change set is limited to repository, notifier, settings, admin template, and version metadata updates and should be covered by existing unit/integration tests if run.
• Manual verification recommended: toggle the new setting, run scans, and confirm notifications contain the change summary and only include added/changed risks when enabled.

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Codex Task