docs-version-sync: cannot resolve the repo root on the public mirror

Describe the bug

The regeneration command documented in docs/AGENTS.md cannot run in this
repository:

$ go run -C tools/docs-version-sync . --target main
docs-version-sync: imports.yaml not found (started from .../tools/docs-version-sync)
exit status 1

findRepoRoot in tools/docs-version-sync/main.go walks up looking only for
imports.yaml, which is intentionally absent on the public mirror. The repo
states this in three places: license-compliance.md:232,
tools/collect-dependencies/scan.go:31, and
tools/collect-dependencies/README.md:38.

The sibling tool tools/collect-dependencies/main.go already handles it by
falling back to the .git marker.

Root AGENTS.md also points agents at imports.yaml without noting that it is
internal, unlike tools/ci/subproject-validations.yaml, which is marked as not
present in this public snapshot.

Steps or code to reproduce bug

go run -C tools/docs-version-sync . --target main

Expected behavior

The documented command runs. Mirror the .git fallback that
collect-dependencies already uses.

Read tools/docs-version-sync/main.go and compare its findRepoRoot logic with tools/collect-dependencies/main.go, then run the documented command from docs/AGENTS.md on the public mirror. Update the root detection and the relevant AGENTS guidance so the command works without imports.yaml; done means the command runs successfully from the mirror.