feat: add cross-tool orientation, CI schema validation, and Claude Code surfaces

Adds the contributor- and AI-tool-facing infrastructure on top of the
modular docs from the previous commit. Lands AGENTS.md as the canonical
cross-tool orientation (read natively by Cursor, Copilot, Codex, Aider,
Cline, Continue, Cody, Devin, Replit Agent, and Claude Code), flips
CLAUDE.md to a 22-line pointer that uses Claude Code's @-import to
inline AGENTS.md, wires up plugin.json validation in CI, and adds the
Claude-specific automation surfaces under .claude/.

Cross-tool orientation:
  AGENTS.md (178 lines) — single source of truth: architecture, running
    the app, testing, git workflow, versioning, song formats, frontend
    and backend conventions, plugin authoring index, first-hour
    pitfalls, verification, house rules.
  CLAUDE.md (22 lines) — Claude Code memory file. Uses @AGENTS.md
    import (recursion depth 5) so the canonical content is inlined
    without duplication. Lists .claude/ surfaces.
  .github/copilot-instructions.md — Copilot custom instructions
    format; points at AGENTS.md and docs/PLUGIN_AUTHORING.md.
  .cursorrules — not added. Cursor reads AGENTS.md natively in 2026
    and .cursorrules is legacy.

Contribution hygiene (.github/):
  PULL_REQUEST_TEMPLATE.md — summary, linked issue, test plan, DCO
    and conventional-commit reminders. No AI-disclosure section.
  ISSUE_TEMPLATE/bug.yml — version, deployment, OS, plugins enabled,
    repro, logs (linked to docs/diagnostics-bundle-spec.md for
    redaction guidance).
  ISSUE_TEMPLATE/feature.yml — problem, proposed, alternatives,
    surface, plugin-author impact, license check.
  ISSUE_TEMPLATE/config.yml — disables blank issues; redirects
    plugin issues to plugin repos and security to the private
    advisory flow.

CI:
  .github/workflows/validate-plugins.yml — runs on changes to
    plugins/*, schema/, CONTRIBUTING.md, the test file, or the
    workflow itself. Installs jsonschema and pytest, validates every
    plugins/*/plugin.json against schema/plugin.schema.json, and runs
    the license-allowlist subset check.
  tests/test_plugin_schema.py — 8 parametrized tests: schema is
    well-formed, the 3 in-tree manifests validate, manifest id
    matches its parent directory name, schema license enum is a
    subset of CONTRIBUTING's curated allowlist.
  requirements-test.txt — append jsonschema>=4.0.
  .github/workflows/sync-version.yml — comment retargeted to
    AGENTS.md "Versioning" section.

Claude Code surfaces (.claude/):
  README.md — layout explanation. Spec-kit owns skills/speckit-*;
    repo-specific skills sit alongside. Hooks off by default;
    settings.json carries a commented opt-in example.
  skills/plugin-scaffold/SKILL.md — generates a plugin skeleton for
    type in {visualization, overlay, settings-only, routes-only}.
  skills/plugin-validate/SKILL.md — local pre-push check: validates
    plugin.json against schema, asserts declared files exist,
    enforces license allowlist.
  rules/plugin-author.md — globs scoped to plugins/**. Encodes the
    contracts from docs/PLUGIN_AUTHORING.md so AI suggestions don't
    drift from them (manifest required, context[\"log\"] over print,
    load_sibling over bare imports, playSong await discipline,
    settings.server_files conventions).
  agents/slopsmith-reviewer.md — plugin-aware reviewer subagent;
    invoke with @slopsmith-reviewer. 12-item checklist mirrors the
    rule and the schema.
  settings.json — empty hooks block plus a commented PostToolUse
    example for opt-in plugin.json validation on save.

Inbound-ref updates (files we own):
  README.md — \"AI Agent Guide\" points at AGENTS.md and notes
    .claude/ and copilot-instructions are tool-specific.
  CONTRIBUTING.md — \"Plugin System in CLAUDE.md\" -> docs/ and
    schema/; \"Git Workflow\" -> AGENTS.md#git-workflow.
  docs/sloppak-spec.md — plugin-system table cell -> docs/.

Out of scope (intentionally untouched):
  plugins/highway_3d/README.md (gitlink — plugin owns its docs).
  .specify/memory/constitution.md and other spec-kit artefacts
    (spec-kit owns that surface; CLAUDE.md still resolves
    transitively via the @-import).

Verification:
  pytest -q                                     # backend + schema tests pass
  python -c \"import json,glob,jsonschema; s=json.load(open('schema/plugin.schema.json')); [jsonschema.validate(json.load(open(p)), s) for p in sorted(glob.glob('plugins/*/plugin.json'))]\"
Signed-off-by: Miguel_LZPF <mgcdreamer@gmail.com>

.github/workflows/sync-version.yml

@@ -3,7 +3,7 @@ name: Sync VERSION from desktop release
 # Updates the VERSION file in this repo whenever slopsmith-desktop
 # publishes a new tagged release. slopsmith-desktop's build.yml
 # dispatches the `desktop-released` event at the end of a successful
-# tag build (see docs in CLAUDE.md). A `workflow_dispatch` trigger is
+# tag build (see Versioning section in AGENTS.md). A `workflow_dispatch` trigger is
 # kept for manual testing / recovery.
 #
 # Related issue: #81.

.github/workflows/validate-plugins.yml

@@ -0,0 +1,70 @@
+name: Validate plugins
+
+on:
+  push:
+    branches: [main]
+    paths:
+      - 'plugins/**/plugin.json'
+      - 'schema/plugin.schema.json'
+      - 'CONTRIBUTING.md'
+      - 'tests/test_plugin_schema.py'
+      - '.github/workflows/validate-plugins.yml'
+  pull_request:
+    branches: [main]
+    paths:
+      - 'plugins/**/plugin.json'
+      - 'schema/plugin.schema.json'
+      - 'CONTRIBUTING.md'
+      - 'tests/test_plugin_schema.py'
+      - '.github/workflows/validate-plugins.yml'
+
+jobs:
+  validate:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+          cache: 'pip'
+
+      - name: Install jsonschema
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements-test.txt
+
+      - name: Validate every in-tree plugin.json against schema
+        run: |
+          python - <<'PY'
+          import glob, json, sys
+          import jsonschema
+
+          schema_path = "schema/plugin.schema.json"
+          with open(schema_path) as f:
+              schema = json.load(f)
+
+          # Validate the schema itself is a well-formed JSON Schema.
+          jsonschema.Draft202012Validator.check_schema(schema)
+
+          manifests = sorted(glob.glob("plugins/*/plugin.json"))
+          if not manifests:
+              print("No plugin manifests found under plugins/*/plugin.json")
+              sys.exit(0)
+
+          failures = []
+          for path in manifests:
+              with open(path) as f:
+                  manifest = json.load(f)
+              try:
+                  jsonschema.validate(manifest, schema)
+                  print(f"OK   {path}")
+              except jsonschema.ValidationError as e:
+                  failures.append((path, e))
+                  print(f"FAIL {path}: {e.message} (at {list(e.absolute_path)})")
+          if failures:
+              sys.exit(1)
+          PY
+
+      - name: Run schema sanity tests
+        run: pytest tests/test_plugin_schema.py -v