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/scaffold/commands/speckit.myext.myextcmd.md

@@ -0,0 +1,20 @@
+---
+description: "Override of the myext extension's myextcmd command"
+---
+
+<!-- Preset override for speckit.myext.myextcmd -->
+
+You are following a customized version of the myext extension's myextcmd command.
+
+When executing this command:
+
+1. Read the user's input from $ARGUMENTS
+2. Follow the standard myextcmd workflow
+3. Additionally, apply the following customizations from this preset:
+   - Add compliance checks before proceeding
+   - Include audit trail entries in the output
+
+> CUSTOMIZE: Replace the instructions above with your own.
+> This file overrides the command that the "myext" extension provides.
+> When this preset is installed, all agents (Claude, Gemini, Copilot, etc.)
+> will use this version instead of the extension's original.