Adopt github.com/apstndb/protoyaml and freeze internal package

[apstndb]

Summary

Two related changes:

1. Adopt the extracted module for the input path. Switch extract.go and
   extract_test.go from the in-repo github.com/apstndb/spannerplan/protoyaml
   helper to the released module github.com/apstndb/protoyaml v0.1.0. The
   YAMLToJSON, UnmarshalJSON, and Unmarshal signatures are identical, so
   this is a pure import swap with zero behavior change — call sites are
   untouched and tests pass with no golden updates. No replace directive is
   used; the module is pulled from the proxy.

2. Freeze the in-repo protoyaml package. Add package-level and
   per-function Deprecated: documentation without changing any behavior. The
   package is retained only for downstream compatibility: spanner-mycli pins the
   exact bytes produced by Marshal. That output format was never a designed
   artifact but an accident of reflection-based encoding; new code should use
   the canonical protojson-over-YAML module. The package is slated for removal
   in v0.3.0, once spanner-mycli has migrated. The repo README mention is
   updated accordingly.

Validation

• go test ./... — pass, zero golden changes
• golangci-lint run ./... — clean (0 issues)

🤖 Generated with Claude Code

https://claude.ai/code/session_01Du9UR1LPdSXk4wAZr4Nf4g

[gemini-code-assist]

Code Review

This pull request deprecates and freezes the internal protoyaml package, replacing its usages in extract.go and extract_test.go with the external github.com/apstndb/protoyaml package. The internal package is marked as deprecated and will be removed in v0.3.0. I have no feedback to provide as there are no review comments.

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