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/copilot-instructions.md

@@ -0,0 +1,21 @@
+# GitHub Copilot instructions
+
+This file customizes GitHub Copilot Chat and Copilot inline suggestions for the Slopsmith repository.
+
+## Read first
+
+- [`AGENTS.md`](../AGENTS.md) — canonical project orientation (architecture, conventions, first-hour pitfalls, verification). This file is a thin pointer; the real content lives there.
+- [`docs/PLUGIN_AUTHORING.md`](../docs/PLUGIN_AUTHORING.md) — plugin work entry point
+
+## House rules
+
+- **License: AGPL-3.0-only.** Inbound contributions are AGPL-compatible. Do not suggest code copied verbatim from incompatible sources.
+- **DCO sign-off required** on every commit (`git commit -s`).
+- **No frontend frameworks.** Vanilla JS, Canvas, Tailwind classes. Do not suggest React/Vue/Svelte additions.
+- **Plugin backend logging.** Suggest `context["log"]`, never `print()`.
+- **Plugin Python imports.** For cross-file backend plugins, suggest `context["load_sibling"]("module_name")` instead of bare `from module_name import X`.
+- **DCO/license headers.** When creating a new file in the main repo, no license header is needed (the LICENSE file at root governs). Plugin authors should add an SPDX-License-Identifier comment to their plugin's source files; the `license` field in `plugin.json` must match the allowlist in [`CONTRIBUTING.md`](../CONTRIBUTING.md).
+
+## Validation
+
+When suggesting changes to a `plugin.json`, validate against [`schema/plugin.schema.json`](../schema/plugin.schema.json).