M0: Freeze v2 config schema and replace ad hoc YAML parsing

[dbrosio3]

Goal

Freeze the v2 .pushgate.yml config schema and replace the current grep/awk-based config reader with a reliable parser and validator.

User value

Users get predictable behavior, useful validation errors, and support for real YAML structures without fragile formatting constraints.

Implementation notes

• Turn the schema decisions left open by the v2 product contract into a versioned .pushgate.yml schema: top-level sections, defaults, deterministic command shape, AI mode fields, and AI provider selection/extension points.
• Validate .pushgate.yml against that versioned schema.
• Keep .push-review.yml compatibility in the migration layer, not as the primary parser or v2 vocabulary.
• Report unknown keys, invalid enum values, missing required fields, and unsafe command shapes.
• Produce typed internal config objects that the deterministic runner and local AI layer can consume without parsing YAML again.

Risks / tradeoffs

• A real parser may introduce a runtime or packaging dependency.
• Schema decisions made here constrain the runner, command-check, and provider tasks that follow.
• Staying pure Bash keeps installation simple but would keep YAML support brittle.

Suggested priority

P0

Milestone

M0: Foundation

TBD considerations

• Runtime/distribution choice: zero-runtime Bash, standalone binary, npm package, Homebrew formula, or hybrid.
• Final deterministic command syntax and provider config shape before runner implementation depends on them.

Acceptance criteria

• The v2 .pushgate.yml schema is versioned and documents the config surface needed by deterministic checks and local AI provider selection.
• Valid v2 config loads consistently into typed internal config.
• Invalid config exits with clear actionable errors.
• Tests cover multiline lists, nested objects, missing keys, invalid enums, comments, unsafe command shapes, and provider-selection validation.