Docs/incident response playbook

[heymide]

Close: #562

All 6 fixes applied and verified:

1. parseMappingTable — simplified to match any line starting with | ` (no more separator detection needed). Handles both |---|---| and | --- | --- | formats.
2. Duplicate oidc_discovery_changed — removed the second entry at line 35 of the script.
3. Missing email_alarm_* entries — added email_alarm_alignment_failure and email_alarm_high_bounce_ratio rows to the README mapping table.
4. fail() → throw new Error() — Jest 30 removed the global fail() function.
5. KNOWN_ALERTS mutation leak — added finally block to restore the array after the mutation test.
6. Verification — npm run validate:alert-mappings exits 0, all 8 Jest tests pass.

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[heymide]

Done, Close: #638