feat: add framework-owned templates

[apcjoaofilho]

What

Adds the M1 framework-owned template foundation under templates/ plus a validator script and tests.

Why

Closes #1

Inline templates cannot be reused programmatically or validated. Dedicated files make the framework easier to version, copy into projects, and check in CI.

How

• Added reusable templates for STATUS.md, STATE.json, project-root AGENTS.md, and ADR.md.
• Added manual-engine fallback templates for spec-delta.md and project.md, both explicitly labelled as manual fallback only because OpenSpec owns native spec artifacts when spec_engine = openspec.
• Added skeletal cron prompt templates for pipeline-manager and single-project runs; M5 will complete the end-to-end cron contract.
• Added scripts/validate-templates.py with --format tty|json, expected-template checks, text placeholder-header checks, manual-fallback labelling checks, and STATE.json schema validation.
• Added tests/templates/test_templates.py covering template presence, placeholder headers, STATE schema validation, manual fallback labels, validator success, and missing-template failure.
• Updated templates/README.md, references/README.md, and CHANGELOG.md.

Scope check

• PR addresses a single concern
• No vendor lock-in introduced outside skill/
• PLAN.md updated if scope changed (new phase/template/script/flavor)
• Tests added/updated where required (see CONTRIBUTING.md)
• CHANGELOG.md entry added under [Unreleased]
• ruff check . passes
• pytest passes (where applicable)

Notes for reviewers

• This PR follows the implementation handoff split: gate YAML templates are left to M6 (M6: Gates as YAML, machine-verifiable #6), where the gate schema and machine-verifiable checks are defined together.
• templates/handoff-schema.yaml is left to M12, where scripts/handoff.py can validate it alongside backend behavior.
• templates/STATE.json cannot start with a comment because it must remain valid JSON and validate against references/state-schema.json; the validator treats it as the only header-comment exception.
• Verification run locally: ruff check ., pytest -q, python scripts/validate-templates.py --format json, python scripts/validate-state.py templates/STATE.json --format json, and git diff --check.