feat: add v2 core templates and validation tooling

[apcjoaofilho]

What

Adds the v2 core artifacts that sit on top of the already-merged STATE schema work: project/state templates, phase gate YAMLs, cron prompt templates, template/status validation tooling, CI, and README wiring for the documentation workflow.

Why

Moves the v2.0 foundation forward against M1, M2, M5, and the template side of M6, while keeping M6 gate consumption in scripts/advance-pipeline.py for the next implementation slice.

How

• Adds framework-owned templates under templates/.
• Adds machine-verifiable gates under templates/gates/.
• Adds scripts/check-status.py and scripts/validate-templates.py.
• Adds tests for status reading, state template/schema validation, and template validation.
• Adds CI workflows for pytest, Ruff, and template validation.
• Updates README and CHANGELOG for the current workflow.

Scope check

• PR addresses a single concern
• No vendor lock-in introduced outside skill/
• PLAN.md updated if scope changed
• Tests added/updated where required
• CHANGELOG.md entry added under [Unreleased]
• ruff check . passes in CI
• pytest passes in CI
• python scripts/validate-templates.py passes in CI

Notes for reviewers

This PR was rebased onto main after references/state-schema.json and scripts/validate-state.py had already landed there. The remaining M6 work is to make scripts/advance-pipeline.py consume the gate files and prove it with an example smoke test.