Add security and privacy documentation

[peyton-alt]

No description provided.

[cursor]

PR Summary

Low Risk
Documentation-only changes with no code or behavior impact; low risk aside from ensuring the guidance stays accurate.

Overview
Adds a new Security & Privacy section to README.md warning that AI session transcripts are committed to the entire/checkpoints/v1 branch and may be publicly visible, and links to a dedicated doc.

Introduces docs/security-and-privacy.md detailing transcript storage, always-on secret redaction (entropy + gitleaks patterns), and practical recommendations/limitations for handling sensitive data.

^{Written by Cursor Bugbot for commit 70a935c. Configure here.}

[Copilot]

Pull request overview

Adds user-facing documentation clarifying how Entire stores session transcripts in git and what secret-redaction safeguards/limitations exist, and links that guidance from the README.

Changes:

• Add docs/security-and-privacy.md describing transcript storage location, redaction approach, and limitations.
• Add a new “Security & Privacy” section in README.md and link it from the TOC.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
docs/security-and-privacy.md	New security/privacy doc covering transcript storage and redaction behavior/limits
README.md	Adds a brief security/privacy summary and link to the full doc