Add label-based Slack channel routing

[smolaon]

Summary

• Route alerts to different Slack channels based on pod label rules (token mode only)
• Each route lists label regexes (all must match, unanchored — use ^...$ to anchor) and one or more target channels; all matching routes apply, channels deduplicated in first-seen order
• Events matching no route, and label-less messages (PVC alerts, startup ping), fall back to the default channel; webhook mode is unchanged and ignores routes with a warning
• Invalid routes config (bad regex, missing labels/channels) fails provider init loudly; per-channel post failures are aggregated with errors.Join and don't block remaining channels
• Composes with the SLACK_TOKEN/SLACK_CHANNEL env fallback (feat(slack): Add environment variable fallback for config #6): credentials come from the Secret-backed env vars, while the non-secret routes stay in the ConfigMap-mounted config file

Config example

alert:
  slack:
    # token/channel may come from SLACK_TOKEN/SLACK_CHANNEL env instead
    channel: "#alerts-default"
    routes:
      - labels:
          app: "^payments-.*"
          tier: "backend"
        channels:
          - "#payments-alerts"
          - "#backend-oncall"

Test plan

• 9 new unit tests: route parsing, init validation failures, webhook-mode ignore, AND-matching/dedup/fallback semantics, multi-channel SendEvent routing, SendMessage default channel, error aggregation
• go build ./..., go vet, package tests green after rebase onto main (includes added support for slack token #3 token support and feat(slack): Add environment variable fallback for config #6 env fallback)
• End-to-end config round-trip verified through the real LoadConfig → NewSlack path
• Manual: run against a dev cluster with a real bot token (token must be invited to each routed channel)

🤖 Generated with Claude Code

[smolaon]

Added includeDefault per-route option (commit 1fe926c): a matching route can also notify the default channel without hardcoding its name — needed for dual-send when the default channel comes from the SLACK_CHANNEL env var. Branch image published as kwatch:exaforce-1fe926c75c2576175b158ff1c76505baf8cbbb06.