feat(export): add slack, pagerduty, and discord webhook sinks

[abhinavuser]

fixes #29

added built-in webhook sinks for kerno doctor findings. wired up the --sink, --severity-floor, and --sink-dedupe flags.
what's included:

• Sink interface with exponential backoff retries on 5xx errors
• in-memory deduplicator to prevent flapping rules from spamming on-call
• slack integration using block kit format for clean readability
• pagerduty integration via events api v2. handles trigger events and automatically sends a resolve event when a finding clears in continuous mode
• discord integration utilizing discord's native slack-compat url suffix
• kerno_sinks_deduped_total and kerno_sinks_failed_total metrics
• full test suite using httptest.Server
  all env vars like KERNO_SLACK_WEBHOOK_URL are expanded automatically if passed as flag values to keep shell history clean.

Signed-off-by: abhinavuser abhinavkumarv2101@gmail.com

[github-actions]

🚀 First PR — welcome aboard!

A few things to expect:

1. CI: every PR runs build + race tests + lint + (eventually) the kernel matrix. If something fails, the log will tell you exactly which gate.
2. DCO: every commit needs Signed-off-by: — git commit -s adds it automatically.
3. Conventional Commits: PR titles like feat(doctor): add new rule or fix(bpf): handle X. We squash-merge by default.
4. Review: a maintainer will review within 72 hours. Suggestions are conversations, not orders — push back if something doesn't fit your context.

If you get stuck, reply here or jump to Discussions. We want this PR to land.

[btwshivam]

dont spam. first get your past pRs merged

[btwshivam]

fix conflict and rebase with main.. then ill review tag me

[abhinavuser]

hey @btwshivam i was waitin for my first pr to get merged thats why the delay but now the rebase is done and the merge conflict in internal/cli/doctor.go is resolved
i also noticed and fixed a small unused import/missing io package issue in sink.go that is causing the CI linter to fail early
ready for your review when you have a chance