fix: resolve attached recurrences again

[ernescz]

Summary

• atomically reopen a resolved incident when a genuine recurrence attaches, allowing the existing one-shot resolution transition to notify again
• keep duplicate resolved deliveries silent and make recurrence re-judgment share the same resolution ownership rule
• serialize per-incident Slack root-card writes so delayed recurrence edits cannot overwrite the resolved card
• add the first Unreleased fix candidate toward application v0.13.8; chart metadata remains unchanged until the coordinated release step

Closes #81

Verification

• GOCACHE=/tmp/alertint-issue81-go-cache go test -race -count=1 ./...
• go build ./...
• go vet ./...
• golangci-lint run ./... (0 issues)
• gosec -quiet ./...
• independent code review: no remaining critical or important findings

Checklist

• Tests were added or updated where behavior changed.
• Documentation was updated where user-facing behavior changed.
• CHANGELOG.md was updated for a notable user-facing change.
• Every commit includes a DCO Signed-off-by trailer.