docs: pre-v0.1.0 polish (README + CHANGELOG + SECURITY)

[bilby91]

Summary

Pre-tag polish that addresses three v0.1.0 milestone issues:

• README — flip six spec-compliance rows that referenced now-closed issues to ✅:
  shutdownAction, initializeCommand, secretsCommand, ${containerEnv:*},
  nested unknown-field warnings, Alpine/BusyBox updateRemoteUserUID.
  Drop the stale test-count line and the link to the gitignored PRD.md.
• CHANGELOG.md — Keep a Changelog format with a single `0.1.0` entry covering
  M1–M5 scope and known limitations.
• SECURITY.md — private vulnerability reporting via GitHub Security
  Advisories, supported versions, response windows, hardening notes for
  embedders.

Closes #4, closes #5.

#3 (move `design/` to public `docs/`) is re-milestoned and retitled to
"write public-facing reference docs in docs/" for a follow-up release —
the current docs are decision logs, not user-facing reference material.

Test plan

• README renders with no broken cross-references
• CHANGELOG link references valid
• No code changes; `go test ./...` unaffected

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: c74382cc-dff3-47fe-a324-9893047ec171

📥 Commits

Reviewing files that changed from the base of the PR and between ab3c43c and 734050e.

📒 Files selected for processing (3)

• CHANGELOG.md
• README.md
• SECURITY.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch pre-v0-1-0-polish

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