Finish droidrun → mobilerun rename (imports + branding)

[linear-code]

Context

The package is now mobilerun, but the codebase, docs, contributor PRs and community forks still reference droidrun (e.g. droidrun.tools..., forks named droidrun). This mid-transition split fragments the brand and complicates rebasing older PRs. It also mirrors the SEO/authority split (link equity + search authority spread across droidrun and mobilerun).

Task

• Audit remaining droidrun references in code, imports, docs, examples, and config.
• Decide + document the canonical brand (droidrun repo/org vs. mobilerun package).
• Ensure github.com/droidrun/droidrun redirects cleanly to droidrun/mobilerun (done) and README/docs are consistent.
• Add a short migration note for contributors so open PRs know the target module paths.

Definition of Done

• No stale droidrun import paths in main
• Docs/README consistent on one brand
• Contributor migration note published

Related: #394*. Also ties into the SEO brand-consolidation finding.*