docs(concepts): schema-format.md + meta-schema.md (#127)

[zeevdr]

Closes #127.

Summary

Graduates the internal `.agents/context/schema-spec.md` design brief into public, maintained user-facing documentation. Two new docs under `docs/concepts/`:

• `schema-format.md` — canonical reference for the `decree.schema.yaml` format. Every top-level key, every field-level option, every constraint with the per-type compatibility matrix, the `dependentRequired` and `validations` cross-field rule mechanisms, the field-path and slug regexes, recommended `format:` values (advisory list), import/export semantics, strict mode, vendor extensions.

• `meta-schema.md` — what the JSON Schema 2020-12 meta-schemas under `schemas/v0.1.0/` cover (layer 1) vs non-goals (layer 2 semantic rules stay Go-side). URL scheme + SemVer + stability policy. How to wire the meta-schema into editor tooling (yaml-language-server modeline) and CI / programmatic validators (check-jsonschema, ajv, santhosh-tekuri/jsonschema).

Reorganization

`schemas-and-fields.md` is trimmed to be the higher-level concepts overview — lifecycle, drafts vs published versions, quick example, brief intros to types and cross-field rules — and cross-links out to `schema-format.md` (full reference) and `meta-schema.md` (mechanical validation). The full format reference content moved to `schema-format.md`.

Other updates

• `docs/getting-started.md` — uses the canonical `decree.schema.yaml` filename + the `# yaml-language-server: $schema=...` modeline on line 1 of the schema example.
• `README.md` — new section under the API table linking to `schema-format.md` + `meta-schema.md` and the published meta-schema URLs.
• `mkdocs.yml` — adds the two new docs to the Concepts nav.
• `typed-values.md` — fixes the orphaned anchor link to point at `schema-format.md#constraints` (the heading moved with the content).

Test plan

• `make validate-meta-schemas` passes (no schema YAMLs touched, but the script runs as a regression check).
• All internal cross-links manually verified to land on existing headings.
• mkdocs nav updated; new docs render cleanly in the Material theme.
• CI "Docs check" should pass — only Markdown changes, no proto/sqlc regeneration needed.

Out of scope (follow-ups)

• Publish meta-schema at schemas.opendecree.io #125 — host the meta-schema at `schemas.opendecree.io/schema/v0.1.0/` (the URL referenced throughout these docs is anticipatory).
• Submit schemastore.org PR for editor integration #126 — submit the schemastore.org PR.

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