docs(flake): annotate KEEP+DEP rationale (standards#102)

[hyperpolymath]

Refs standards#101 standards#102.

Per the Nix-mirror retirement campaign (Wave 4), boj-server's flake is LOAD-BEARING (KEEP+DEP): guix.scm declares empty (native-inputs (list)) / (inputs (list)) and acknowledges "idris2 and zig packages may need custom channels". This flake's devShell is therefore the sole source of the language layer (idris2, zig, zls) and the RSR-template common dev tools (git, just, nickel, curl, bash, coreutils).

This PR adds the rule-3 annotation so the flake's retention is self-documenting. No package-set change. Orthogonal to in-flight boj-server#73 (HCG tier-2 mTLS) work.

🤖 Generated with Claude Code

[github-actions]

🔍 Hypatia Security Scan

Findings: 29 issues detected

Severity	Count
🔴 Critical	18
🟠 High	4
🟡 Medium	7

⚠️ Action Required: Critical security issues found!

View findings

[
  {
    "reason": "Stale AI session file -- delete",
    "type": "stale",
    "file": "GEMINI.md",
    "action": "delete",
    "rule_module": "root_hygiene",
    "severity": "medium"
  },
  {
    "reason": "Issue in quality.yml",
    "type": "missing_workflow",
    "file": "quality.yml",
    "action": "create",
    "rule_module": "workflow_audit",
    "severity": "high"
  },
  {
    "reason": "Issue in security-policy.yml",
    "type": "missing_workflow",
    "file": "security-policy.yml",
    "action": "create",
    "rule_module": "workflow_audit",
    "severity": "medium"
  },
  {
    "reason": "Action hyperpolymath/standards/.github/workflows/governance-reusable.yml@main needs attention",
    "type": "unpinned_action",
    "file": "governance.yml",
    "action": "pin_sha",
    "rule_module": "workflow_audit",
    "severity": "high"
  },
  {
    "reason": "Python file detected -- banned language",
    "type": "banned_language_file",
    "file": "/home/runner/work/boj-server/boj-server/.github/scripts/validate-eclexiaiser.py",
    "action": "flag",
    "rule_module": "cicd_rules",
    "severity": "critical"
  },
  {
    "reason": "TypeScript file detected -- banned language",
    "type": "banned_language_file",
    "file": "/home/runner/work/boj-server/boj-server/cartridges/sanctify-mcp/adapter/mod.ts",
    "action": "flag",
    "rule_module": "cicd_rules",
    "severity": "critical"
  },
  {
    "reason": "TypeScript file detected -- banned language",
    "type": "banned_language_file",
    "file": "/home/runner/work/boj-server/boj-server/cartridges/academic-workflow-mcp/adapter/mod.ts",
    "action": "flag",
    "rule_module": "cicd_rules",
    "severity": "critical"
  },
  {
    "reason": "TypeScript file detected -- banned language",
    "type": "banned_language_file",
    "file": "/home/runner/work/boj-server/boj-server/cartridges/fireflag-mcp/adapter/mod.ts",
    "action": "flag",
    "rule_module": "cicd_rules",
    "severity": "critical"
  },
  {
    "reason": "TypeScript file detected -- banned language",
    "type": "banned_language_file",
    "file": "/home/runner/work/boj-server/boj-server/cartridges/ephapax-mcp/adapter/mod.ts",
    "action": "flag",
    "rule_module": "cicd_rules",
    "severity": "critical"
  },
  {
    "reason": "TypeScript file detected -- banned language",
    "type": "banned_language_file",
    "file": "/home/runner/work/boj-server/boj-server/cartridges/bofig-mcp/adapter/mod.ts",
    "action": "flag",
    "rule_module": "cicd_rules",
    "severity": "critical"
  }
]

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