feat(core): consolidate skills config, ship JSON schema, simplify gitignore

[rgdevme]

Summary

• Consolidate skills config under agnos.json#skills. The skills map and paths.skillsDir are now a single block: skills: { route, sources }, mirroring how docs is configured. One top-level key per domain, no more fragmentation.
• Ship a JSON schema for agnos.json. New packages/core/schema.json (Draft 2020-12) covers the full config surface — agents, rules, skills, mcp, docs. The default $schema (and agnos init backfill on existing projects) points at https://unpkg.com/@luxia/core/schema.json so editors get autocomplete + validation as soon as the next version is published.
• Simplify the gitignore. agnos init now writes a single .agnos/ line instead of .agnos/cache/ + .agnos/state.json. Everything under .agnos/ is reproducible from agnos.json + agnos.lock.json.

Breaking change

agnos.json shape has changed. Before:

```json
{
"skills": { "pdf": "github:..." },
"paths": { "skillsDir": ".agnos/skills" }
}
```

After:

```json
{
"skills": {
"route": ".agnos/skills",
"sources": { "pdf": "github:..." }
}
}
```

Existing projects will need to move their skills map under skills.sources and (if customized) paths.skillsDir under skills.route manually. No auto-migration is written — flag if you want one added before merge.

Notable code touchpoints

• Schema/types: `packages/core/src/schema.ts`, `packages/core/src/types/public.ts` (new `SkillsConfig` interface; removed `PathsConfig`).
• Readers: `paths.ts` reads `config.skills?.route`; `orchestrator.ts`, `skill-prepare.ts`, and `commands/skill.ts` iterate `config.skills?.sources`.
• Public exports: `SkillsConfig`, `skillSourcesSchema`, `SCHEMA_URL` added; `PathsConfig` / `pathsConfigSchema` removed.
• `commands/init.ts`: `ensureGitIgnore` writes one line; new `backfillSchemaField` sets `$schema` on existing configs.
• `packages/core/package.json` `files` includes `schema.json` so it ships in the published tarball.

Test plan

• `pnpm -r typecheck` clean across all 8 packages
• `pnpm -r test` — 155/155 passing (17 + 1 + 6 test files in core / domain-skills / domain-docs)
• Hand-validated example JSON in README parses against the new schema via `ajv-cli` (draft 2020); invalid inputs (non-string agent ids, illegal skill names) correctly rejected
• Smoke-test `agnos init` against a fresh project once published — confirm `$schema` lands and `.agnos/` gitignore line is correct
• Confirm IDE schema fetch works once `@luxia/core@0.0.7+` is on npm with `schema.json` in the tarball