feat: add v2 config schema validation

[dbrosio3]

Description

Closes #2.

Adds the v2 .pushgate.yml config contract and a typed config loader so Pushgate can validate and normalize project config before later deterministic runner and AI provider layers consume it.

This replaces ad hoc YAML parsing assumptions with a versioned schema, actionable validation errors, explicit legacy-file handling, and a config shape that can safely support nested provider settings and argv-based tool commands.

Type of change

• fix: - Bug fix (patch version bump)
• feat: - New feature (minor version bump)
• feat!: / fix!: - Breaking change (major version bump)
• docs: - Documentation only (no version bump)
• chore: - Maintenance or config (no version bump)
• refactor: - Code restructure, no behaviour change (no version bump)

Changes made

• Add a versioned v2 JSON Schema and TypeScript config loader for .pushgate.yml.
• Normalize defaults for review, AI mode, tools, and ignored paths into typed internal config objects.
• Validate invalid versions, unknown keys, unsafe tool command shapes, invalid AI modes, and provider-selection mismatches with actionable errors.
• Surface migration behavior for legacy .push-review.yml repos, including legacy-only failures and coexistence warnings.
• Update bundled templates, README examples, and schema docs to use version: 2, nested ai.providers config, argv-array tool commands, and review prompt defaults outside the config surface.
• Add config fixtures and tests covering valid configs, defaults, schema failures, provider selection, legacy config behavior, and bundled template compatibility.
• Add the Node config-layer build/test setup and CI path using pnpm and Corepack.

Testing

• pnpm install --frozen-lockfile
• pnpm build
• pnpm test
• bash -n hook/pre-push passes with no output
• bash -n install.sh passes with no output
• Manually tested the hook on a real repository
• Tested on macOS
• Tested on Linux

Checklist

• Commit messages follow Conventional Commits
• No eval, heredoc variable expansion, or unquoted variable interpolation in shell scripts
• File lists passed as arrays, never as interpolated strings
• VERSION, CHANGELOG.md, .release-please-manifest.json, and release-please-config.json were not manually edited - these are managed by release-please
• New template added to the table in README.md (not applicable)
• New template name added to install.sh usage comment (not applicable)

Screenshots / output

N/A. This PR adds the v2 config contract, validation layer, docs, templates, and automated coverage.