chore: clean up the tree (embed updateUID Dockerfile, drop dead assets)

Reviewing the working tree for cruft:

- Embed scripts/updateUID.Dockerfile into the binary (`//go:embed`, moved to
  internal/docker/) and build it from a temp context. It was previously read
  from disk at runtime via a path search next to the executable — which silently
  SKIPPED the UID remap for any distributed/`go install`ed binary that did not
  ship scripts/. Now the single static binary is truly self-contained.
- Delete scripts/gitAskPass.sh — unreferenced anywhere; the scripts/ directory is
  gone entirely.
- Delete three orphaned test fixtures with zero references from the parity matrix
  or any test: src/test/configs/{disallowed-features, dockerfile-with-syntax,
  poetry-example}.

Verified: build + full unit suite + golangci-lint (0 issues) green; the embedded
Dockerfile is present in the binary.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: aledbf