docs: Update docs, add acknowledgements

[Pringled]

A few docs changes from #157 were lost, this PR restores them. Also added an acknowledgements section thanking Greptile for their free access to their platform. Lastly, added a bit more info on caching/invalidation to the how it works section (see #163).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[greptile-apps]

Confidence Score: 5/5

Documentation-only change with no code modifications; safe to merge.

All changes are confined to README.md. The removed Bash/AGENTS.md section and its promoted Sub-agent setup heading are straightforward restructuring. The updated caching description and new Acknowledgements section introduce no logic, build, or runtime risk.

No files require special attention.

_{Reviews (2): Last reviewed commit: "Update docs" | Re-trigger Greptile}