governance: enforce DCO and document continuity checks

[TFT444]

Summary

Implements the repository-controlled parts of #205.

• adopts DCO 1.1 as the contribution authorization mechanism
• adds a dependency-free PR check requiring a valid Signed-off-by trailer on every introduced commit
• documents contributor sign-off and repair instructions
• adds the DCO requirement to the pull request checklist
• adds an owner-verifiable continuity capability matrix and six-month review process
• adds regression tests for valid, missing, malformed, and mixed commit sign-offs

Validation

• 3 DCO helper tests pass
• this PR commit passes the new DCO checker
• Ruff and formatting pass
• workflow YAML parses successfully
• git diff --check passes

Owner confirmation still required

Issue #205 must remain open until an organization owner privately verifies that at least two people can administer issues, merge approved work, publish/recover releases, and recover critical organization/deployment access. Documentation cannot prove that underlying access state.

Advances #205

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

OpenSSF Scorecard

Package	Version	Score	Details
actions/actions/checkout	34e114876b0b11c390a56381ad16ebd13914f8d5	🟢 7	Details Check Score Reason Code-Review 🟢 10 all changesets reviewed Maintained 🟢 10 20 commit(s) and 0 issue activity found in the last 90 days -- score normalized to 10 Dangerous-Workflow 🟢 10 no dangerous workflow patterns detected Binary-Artifacts 🟢 10 no binaries found in the repo Token-Permissions ⚠️ 0 detected GitHub workflow tokens with excessive permissions CII-Best-Practices ⚠️ 0 no effort to earn an OpenSSF best practices badge detected Fuzzing ⚠️ 0 project is not fuzzed License 🟢 10 license file detected Pinned-Dependencies 🟢 3 dependency not pinned by hash detected -- score normalized to 3 Packaging ⚠️ -1 packaging workflow not detected Signed-Releases ⚠️ -1 no releases found Security-Policy 🟢 9 security policy file detected SAST 🟢 10 SAST tool is run on all commits Branch-Protection 🟢 6 branch protection is not maximal on development and all release branches

Scanned Files

• .github/workflows/dco.yml