docs: fix misc remaining docs (architecture, dev quickref, sub-dir READMEs)

PLUGIN_ARCHITECTURE_SPEC.md
- Added a banner at the top noting this is a historical design doc
  written before the plugin system shipped. The doc is ~1900 lines
  with 13 stale /api/plugins/* paths (real is /api/v3/plugins/*),
  references to web_interface_v2.py (current is app.py), and a
  Migration Strategy / Implementation Roadmap that's now history.
  Banner points readers at the current docs
  (PLUGIN_DEVELOPMENT_GUIDE, PLUGIN_API_REFERENCE,
  REST_API_REFERENCE) without needing to retrofit every section.

PLUGIN_CONFIG_ARCHITECTURE.md
- 10 occurrences of /api/plugins/* missing /v3 prefix. Bulk fixed.

DEVELOPER_QUICK_REFERENCE.md
- cache_manager.delete("key") -> cache_manager.clear_cache("key")
  with comment noting delete() doesn't exist. Same bug already
  documented in PLUGIN_API_REFERENCE.md.

SSH_UNAVAILABLE_AFTER_INSTALL.md
- 4 occurrences of port 5001 -> 5000 in AP-mode and Ethernet/WiFi
  recovery instructions.

PLUGIN_CUSTOM_ICONS_FEATURE.md
- Port 5001 -> 5000.

CONFIG_DEBUGGING.md
- Documented /api/v3/config/plugin/<id> and /api/v3/config/validate
  endpoints don't exist. Replaced with the real endpoints:
  /api/v3/config/main, /api/v3/plugins/schema?plugin_id=,
  /api/v3/plugins/config?plugin_id=. Added a note that validation
  runs server-side automatically on POST.

STARLARK_APPS_GUIDE.md
- "Plugins -> Starlark Apps" UI navigation path doesn't exist (5
  occurrences). Replaced with the real path: Plugin Manager tab,
  then the per-plugin Starlark Apps tab in the second nav row.
- "Navigate to Plugins" install step -> Plugin Manager tab.

web_interface/README.md
- Documented several endpoints that don't exist in the api_v3
  blueprint:
  - GET /api/v3/plugins (list) -> /api/v3/plugins/installed
  - GET /api/v3/plugins/<id> -> doesn't exist
  - POST /api/v3/plugins/<id>/config -> POST /api/v3/plugins/config
  - GET /api/v3/plugins/<id>/enable + /disable -> POST /api/v3/plugins/toggle
  - GET /api/v3/store/plugins -> /api/v3/plugins/store/list
  - POST /api/v3/store/install/<id> -> POST /api/v3/plugins/install
  - POST /api/v3/store/uninstall/<id> -> POST /api/v3/plugins/uninstall
  - POST /api/v3/store/update/<id> -> POST /api/v3/plugins/update
  - POST /api/v3/display/start/stop/restart -> POST /api/v3/system/action
  - GET /api/v3/display/status -> GET /api/v3/system/status
- Also fixed config/secrets.json -> config/config_secrets.json
- Replaced the per-section endpoint duplication with a current real
  endpoint list and a pointer to docs/REST_API_REFERENCE.md.
- Documented that SSE stream endpoints are defined directly on the
  Flask app at app.py:607-615, not in the api_v3 blueprint.

scripts/install/README.md
- Was missing 3 of the 9 install scripts in the directory:
  one-shot-install.sh, configure_wifi_permissions.sh, and
  debug_install.sh. Added them with brief descriptions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/PLUGIN_ARCHITECTURE_SPEC.md

@@ -1,5 +1,24 @@
 # LEDMatrix Plugin Architecture Specification
 
+> **Historical design document.** This spec was written *before* the
+> plugin system was built. Most of it is still architecturally
+> accurate, but specific details have drifted from the shipped
+> implementation:
+>
+> - Code paths reference `web_interface_v2.py`; the current web UI is
+>   `web_interface/app.py` with v3 Blueprint-based templates.
+> - The example Flask routes use `/api/plugins/*`; the real API
+>   blueprint is mounted at `/api/v3` (`web_interface/app.py:144`).
+> - The default plugin location is `plugin-repos/` (configurable via
+>   `plugin_system.plugins_directory`), not `./plugins/`.
+> - The "Migration Strategy" and "Implementation Roadmap" sections
+>   describe work that has now shipped.
+>
+> For the current system, see:
+> [PLUGIN_DEVELOPMENT_GUIDE.md](PLUGIN_DEVELOPMENT_GUIDE.md),
+> [PLUGIN_API_REFERENCE.md](PLUGIN_API_REFERENCE.md), and
+> [REST_API_REFERENCE.md](REST_API_REFERENCE.md).
+
 ## Executive Summary
 
 This document outlines the transformation of the LEDMatrix project into a modular, plugin-based architecture that enables user-created displays. The goal is to create a flexible, extensible system similar to Home Assistant Community Store (HACS) where users can discover, install, and manage custom display managers from GitHub repositories.
@@ -9,7 +28,7 @@ This document outlines the transformation of the LEDMatrix project into a modula
 1. **Gradual Migration**: Existing managers remain in core while new plugin infrastructure is built
 2. **Migration Required**: Breaking changes with migration tools provided
 3. **GitHub-Based Store**: Simple discovery system, packages served from GitHub repos
-4. **Plugin Location**: `./plugins/` directory in project root
+4. **Plugin Location**: `./plugins/` directory in project root *(actual default is now `plugin-repos/`)*
 
 ---