docs: add case study

[swares]

Adds docs/CASE-STUDY.md, written for a hiring manager rather than an operator:

• What it is, in one table, plus the design decisions worth explaining.
• Failures that reported success: the offsite backup that passed while copying nothing, the restore drill that would have false-passed, the seven-week silent Alloy pipeline, sshd -T vs. the playbook (and the lockout that followed), the 37-day invisible nfs-server, and the Vault snapshot gap whose alert fired to a stolen phone.
• Two outages: the DNS wildcard and the H4 boot deadlock.
• The habits that came out of them, and open gaps linked to BACKLOG.

Every fact is taken from BACKLOG.md, CLAUDE.md or the incident record; nothing new is claimed. The README gains a link and a repo-map row.

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https://claude.ai/code/session_01SxKgLH9zdFjEYXWaceikdk

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