Repo hygiene: prune root markdown bloat; revise credentials guidance

[gmoon]

Problem

The repo root contains 10 markdown files totaling ~3,200 lines. Most are process artifacts (release guides, migration post-mortems, AI-development rules) that bloat the repo without informing contributors.

The README documents a workflow that drops short-lived AWS STS credentials in the repo root as credentials.json. The file is .gitignored (correctly), but the pattern depends on a single line of defense across an evolving file and instructs forks to repeat the pattern.

Acceptance criteria

• Delete from repo root (move to git history): MIGRATION_SUMMARY.md, RELEASE.md, RELEASE_INSTRUCTIONS.md, RELEASE_QUICK_REFERENCE.md, TODO.md, AI_ASSISTED_DEVELOPMENT_RULES.md, S3PROXY_DOCKER_ADAPTATION_GUIDE.md, load-test-results-*.json.
• Move MAINTENANCE.md and PERFORMANCE.md content into README sections or docs/ (decide based on length).
• Update README's credentials section: replace "drop credentials.json in repo root" with "place at ~/.s3proxy/credentials.json, bind-mount from there." Keep the file .gitignored.
• Do not delete credentials.json itself — it's intentional dev tooling for short-lived Docker testing. Leave it alone.
• All existing tests pass.

Notes

Independent of the API refactor.