BUILD-12282: Remove obsolete Cirrus-to-GitHub migration guide

[matemoln]

The Cirrus CI migration is complete, so this guide is no longer needed. Removing it also unblocks default-branch pre-commit run --all-files (MD060).

Summary

• Delete .cursor/cirrus-github-migration.md
• Drop the README and CONTRIBUTE sections that pointed at that guide

Test plan

• pre-commit run --all-files is green
• Pre-commit job is green on this PR
• No remaining references to .cursor/cirrus-github-migration.md

[hashicorp-vault-sonar-prod]

BUILD-12282

[gitar-bot]

Code Review ✅ Approved

Fixes markdownlint table formatting in .cursor/cirrus-github-migration.md to normalize column style with compact pipes and spacing, unblocking pre-commit run --all-files. No issues found.

Review coverage

Functional validation 0 of 5 objectives covered

Rules No rules evaluated

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Implementation Status ◻️ 0 of 5 objectives covered

◻️ BUILD-12282 - 0 of 5 objectives covered

This PR removes the obsolete Cirrus-to-GitHub migration guide, while the diff does not implement the pip/npm Repox routing, nodeenv configuration, workflow permissions, or README updates described in the issue.

Other objectives on this issue, possibly covered elsewhere:

• ◻️ Route pip and npm hook installs through Repox from inside the action without nested config-pip or config-npm
• ◻️ Ensure nodeenv continues to use https://nodejs.org/dist/
• ◻️ Configure action IT and self-pre-commit workflows with id-token: write
• ◻️ Ensure it-tests-repox-runner covers a sonar-xs runner
• ◻️ Document required permissions and covered registries in the README

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues
0 New dependency risks

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud