docs: seed CHANGELOG.md (Keep-a-Changelog format)

[hyperpolymath]

Summary

Adds an initial CHANGELOG.md in Keep-a-Changelog format. Recent commits are bucketed into Added/Fixed/Changed/Documentation/CI from their conventional-commit prefixes.

Closes Row-2 Phase 3 of the 2026-05-26 estate tech-debt audit chain for this repo. The audit (standards#197) flagged 180/279 repos missing CHANGELOG (65% gap).

Adopting auto-regeneration

For ongoing auto-regeneration, adopt the changelog-reusable.yml workflow (standards#206) in this repo's CI. Uses the canonical templates/cliff.toml.

🤖 Generated with Claude Code

[github-actions]

🔍 Hypatia Security Scan

Findings: 23 issues detected

Severity	Count
🔴 Critical	0
🟠 High	15
🟡 Medium	8

View findings

[
  {
    "reason": "Issue in mirror.yml",
    "type": "missing_workflow",
    "file": "mirror.yml",
    "action": "create",
    "rule_module": "workflow_audit",
    "severity": "high"
  },
  {
    "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": "Nominal-only SAST in a2ml_gleam: codeql.yml language matrix contains no language present in the repo and lacks `actions`, so CodeQL records zero results on every commit. Remediation: set the CodeQL matrix to `language: actions`.",
    "type": "StaticAnalysis",
    "file": "/home/runner/work/a2ml_gleam/a2ml_gleam",
    "action": "auto_fix",
    "rule_module": "scorecard",
    "severity": "medium",
    "remediation": "Add CodeQL or equivalent SAST workflow.",
    "scorecard_check": "SAST"
  },
  {
    "reason": "Repository has 6 non-main remote branch(es). Policy: single main branch only.",
    "type": "GS007",
    "file": ".",
    "action": "delete_remote_branches",
    "rule_module": "git_state",
    "severity": "medium"
  },
  {
    "reason": "Code scanning (Scorecard): TokenPermissionsID -- Token-Permissions -- 0 day(s) old",
    "type": "CSA001",
    "file": ".github/workflows/hypatia-scan.yml",
    "action": "update",
    "rule_module": "code_scanning_alerts",
    "severity": "high"
  },
  {
    "reason": "Code scanning (Hypatia): hypatia/git_state/GS007 -- Hypatia git_state: GS007 -- 9 day(s) old",
    "type": "CSA001",
    "file": ".",
    "action": "review",
    "rule_module": "code_scanning_alerts",
    "severity": "medium"
  },
  {
    "reason": "Code scanning (Hypatia): hypatia/scorecard/StaticAnalysis -- Hypatia scorecard: StaticAnalysis -- 9 day(s) old",
    "type": "CSA001",
    "file": "a2ml_gleam",
    "action": "review",
    "rule_module": "code_scanning_alerts",
    "severity": "medium"
  },
  {
    "reason": "Code scanning (Hypatia): hypatia/workflow_audit/unpinned_action -- Hypatia workflow_audit: unpinned_action -- 9 day(s) old [STALE]",
    "type": "CSA001",
    "file": "governance.yml",
    "action": "escalate",
    "rule_module": "code_scanning_alerts",
    "severity": "high"
  },
  {
    "reason": "Code scanning (Hypatia): hypatia/workflow_audit/missing_workflow -- Hypatia workflow_audit: missing_workflow -- 9 day(s) old [STALE]",
    "type": "CSA001",
    "file": "mirror.yml",
    "action": "escalate",
    "rule_module": "code_scanning_alerts",
    "severity": "high"
  },
  {
    "reason": "Code scanning (Scorecard): PinnedDependenciesID -- Pinned-Dependencies -- 9 day(s) old",
    "type": "CSA001",
    "file": ".github/workflows/governance.yml",
    "action": "review",
    "rule_module": "code_scanning_alerts",
    "severity": "medium"
  }
]

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