docs: refresh pre-alpha review findings

[vessenes]

Motivation

• Refresh the pre-alpha security/crypto/code-quality review to record current findings, include environment limitations encountered during validation, and provide a concise set of recommended next actions.

Description

• Update PRE_ALPHA_REVIEW.md to add a What I ran section, an Environment limitations section, rework findings into High/Medium/Pass categories (gate admin auth default, credential handling, dead/stubbed CLI paths, v1.1 artifacts presence), and to list actionable recommendations.

Testing

• Attempted go test ./... (automated): failed due to network/proxy restrictions when fetching Go modules (setup failure).
• Attempted uvx showboat --help (automated): failed due to inability to reach PyPI from this environment.
• Static repository scans using rg (automated): succeeded and were used to verify presence of conv_epoch, group_rekey, handle_reveal, DeriveNonce HMAC usage, TODO/not-implemented markers, and secret-like patterns.

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Codex Task