decree-docs: add doc model, schema loaders, and JSON output

[zeevdr]

Summary

• The architectural core of decree-docs (epic Epic: decree-docs — advanced schema docs generator #921): a strongly-typed docmodel package that supersets the minimal sdk/tools/docgen schema (info, named examples, externalDocs, version_description, allowed_schemes — no untyped fields), so every later backend renders from one complete model.
• The JSON shape is the contract for third-party renderers (the goal stated in Publish meta-schema for schema YAML structure (v0.1.0) #117): lowerCamel keys, omitempty optionals, path-sorted fields, and a docModelVersion root marker, all documented in the package doc and pinned by golden fixtures plus a canonical-form test.
• Loaders populate the model from a schema YAML (via sdk/tools/validate) and from a server (via a narrow loader.SchemaClient interface that *adminclient.Client satisfies); both canonicalize so the equivalence guarantee is structural, not fixture luck. Two drift tests mirror cmd/decree's: every adminclient and every file-format property must be mapped or explicitly listed as not-documented.
• Server-only fields (ID, ParentVersion, Checksum, Published, CreatedAt) are excluded from the model — schema YAML cannot express them, and including them would force the file/server equivalence test to zero them, weakening the guarantee.
• CLI: decree-docs generate --file <schema> --format json; server-mode dialing flags are deferred to decree-docs: config file and template override system #918 (config surface) — the scaffold's dependency footprint deliberately excludes grpctransport until then.

Test plan

• make test — all modules pass, including the three new packages
• ./scripts/check-coverage.sh — contrib/decree-docs 99.1%, ratchet floor raised 88.8 → 99.0
• make lint-go clean; gofumpt -l empty; go build/go vet clean
• Golden JSON fixtures (full + minimal schemas) and loader equivalence test (file vs fake-transport server load, deep-equal)
• Binary E2E: generate --file testdata/full.schema.yaml --format json exits 0, byte-identical to the golden

Closes #914

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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