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>

symphony/skills/symphony-init/SKILL.md

@@ -89,7 +89,7 @@ Keep the interview short. Anything that has a sensible default should default.
    If the project's CLAUDE.md doesn't exist yet, create it with just this section.
    Do not invent other content.
 
-2. **`./WORKFLOW.md`** — copy from the plugin's `templates/WORKFLOW.md`, with
+2. **`./WORKFLOW.md`** — copy from `${CLAUDE_PLUGIN_ROOT}/templates/WORKFLOW.md`, with
    detected values substituted (tracker.kind, active_states, terminal_states,
    concurrency cap, hook commands appropriate to the project's build system).
    If a `WORKFLOW.md` already exists, show the diff and ask before overwriting.