feat(deduplication): add occurrence counter to duplicate comments

[Naoray]

Summary

• Adds occurrence counter tracking to the deduplication system so each comment on a duplicate issue shows how many times the error has been seen (e.g., **Occurrence:** #5)
• The counter uses a separate cache key ({prefix}count:{signature}) from the deduplication signature, increments on every encounter (even deduplicated ones), and refreshes its TTL on each increment
• New config option deduplication.track_occurrences (default: true) allows disabling the feature

Changes

• CacheManager -- Added incrementOccurrenceCount() and getOccurrenceCount() methods with a dedicated count cache key segment
• DeduplicationHandler -- Tracks occurrences via new trackOccurrences constructor parameter; passes count through extra['github_occurrence_count']
• TemplateRenderer -- Added {occurrence_count} replacement (defaults to 1 when not present); filters github_occurrence_count from the Extra Data section
• comment.md -- Added **Occurrence:** #{occurrence_count} to the comment header
• GithubIssueHandlerFactory -- Passes track_occurrences config to DeduplicationHandler
• config/github-monolog.php -- Added deduplication.track_occurrences option

Test plan

• CacheManager: occurrence count increments correctly across calls
• CacheManager: occurrence count uses separate cache key from dedup signature
• CacheManager: different signatures tracked independently
• DeduplicationHandler: first occurrence shows count #1
• DeduplicationHandler: count increments for deduplicated records within TTL
• DeduplicationHandler: count resets after TTL expires
• DeduplicationHandler: count not added when trackOccurrences is false
• TemplateRenderer: occurrence count renders in comment template
• TemplateRenderer: defaults to #1 when no count in extra
• TemplateRenderer: count not leaked into Extra Data section
• Factory: enables tracking by default, respects config override
• All 291 tests pass, PHPStan clean, Pint clean

Closes #40

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