fix(composer,symphony): use \${CLAUDE_PLUGIN_ROOT} for plugin-internal paths

Per plugin-structure spec, plugin-internal references in component
files (commands, agents, skills) must use \${CLAUDE_PLUGIN_ROOT} so
they resolve correctly regardless of where the plugin is installed
(local repo, marketplace cache, npm-installed, etc.).

Updated:
- symphony/skills/symphony-init/SKILL.md: WORKFLOW.md template ref
- composer/skills/ui-system/SKILL.md: all six template refs in the
  Write phase
- composer/skills/composer/SKILL.md: added a Templates section that
  enumerates each phase's template path with \${CLAUDE_PLUGIN_ROOT},
  plus the ADR template ref in phase 8

Composer's CLAUDE.md gains a clarifying note that templates/design-
system/SKILL.md and templates/interaction-design/SKILL.md are template
skeletons, not actual plugin skills — they're copied into a consuming
project's .claude/skills/ at install-time of the design system. Per
the auto-discovery rule (scans skills/ only), they are not loaded as
plugin skills.

Removed empty composer/agents/ directory (composer has no agents;
workers are owned by symphony).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

composer/CLAUDE.md

@@ -48,6 +48,19 @@ not a setting picked at the start. Stack choices made before the design
 system, interaction model, and mocks lock you into shapes that don't fit
 the problem.
 
+## Note on templates
+
+`templates/design-system/SKILL.md` and `templates/interaction-design/SKILL.md`
+are **templates**, not actual skills of this plugin. The plugin's auto-loaded
+skills live in `skills/` (`composer`, `composer-mocks`, `ui-system`); the
+templates under `templates/` are skeletons that the `ui-system` skill copies
+into a *consuming project's* `.claude/skills/<slug>-design-system/` and
+`.claude/skills/<slug>-interaction-design/` directories.
+
+Per Claude Code's plugin auto-discovery, only `SKILL.md` files under `skills/`
+are loaded as plugin skills — `templates/` is a regular subdirectory and is
+ignored by discovery.
+
 ## Layout
 
 - `commands/composer.md` — main entry, state-aware. Continues from the first