feat(presets): pluggable preset system with template/command overrides, catalog, and resolver

- Rename 'template packs' to 'presets' to avoid naming collision with core templates
- PresetManifest, PresetRegistry, PresetManager, PresetCatalog, PresetResolver in presets.py
- Extract CommandRegistrar to agents.py as shared infrastructure
- CLI: specify preset list/add/remove/search/resolve/info
- CLI: specify preset catalog list/add/remove
- --preset option on specify init
- Priority-based preset stacking (--priority, lower = higher precedence)
- Command overrides registered into all detected agent directories (17+ agents)
- Extension command safety: skip registration if target extension not installed
- Multi-catalog support: env var, project config, user config, built-in defaults
- resolve_template() / Resolve-Template in bash/PowerShell scripts
- Self-test preset: overrides all 6 core templates + 1 command
- Scaffold with 4 examples: core/extension template and command overrides
- Preset catalog (catalog.json, catalog.community.json)
- Documentation: README.md, ARCHITECTURE.md, PUBLISHING.md
- 110 preset tests, 253 total tests passing

presets/catalog.json

@@ -0,0 +1,25 @@
+{
+  "schema_version": "1.0",
+  "updated_at": "2026-03-10T00:00:00Z",
+  "catalog_url": "https://raw.githubusercontent.com/github/spec-kit/main/presets/catalog.json",
+  "presets": {
+    "self-test": {
+      "name": "Self-Test Preset",
+      "description": "A preset that overrides all core templates for testing purposes",
+      "author": "github",
+      "version": "1.0.0",
+      "repository": "https://github.com/github/spec-kit",
+      "license": "MIT",
+      "requires": {
+        "speckit_version": ">=0.1.0"
+      },
+      "provides": {
+        "templates": 6,
+        "commands": 1
+      },
+      "tags": ["testing", "self-test"],
+      "created_at": "2026-03-10T00:00:00Z",
+      "updated_at": "2026-03-10T00:00:00Z"
+    }
+  }
+}