docs(security): split git-backup threat model into its own page

[fdaviddpt]

docs(security): split git-backup threat model into its own page

Context

The README "Trust Model" section reads like a CVE advisory and applies all three threats to every install — even though git backup is opt-in. Scares off users who'd never enable backup and don't carry that risk.

Changes

• README.md — replace the three-bullet trust block with a short paragraph: default install adds no new attack surface beyond Claude Code itself; one-line pointer to the detailed doc for users who opt into git backup.
• docs/git-backup-security.md (new) — full threat model scoped to the opt-in feature. Leads with the disarmer: ~/.remember/ has the same trust assumptions as ~/.ssh/, ~/.bashrc, ~/.gitconfig — if an attacker can write your home dir as your user, the plugin is the least of your worries. Then the three real concerns specific to git backup, and the recommended setup.

No code changes, no behavior changes.

Why this framing

• The original list conflated "this plugin is dangerous" with "any home-dir-writable file is dangerous". The second is true and universal; surfacing it disarms the first.
• Most users will never enable git backup. They shouldn't have to read a threat model for a feature they don't use.
• The mitigations the plugin already has (remote-URL validation + allow_remote_change) deserve more prominence than they got in the original block.