Phase D: cross-runtime tweaks (visual-companion, executing-plans, test)

Misc platform/runtime statements and adjacencies that don't fit the
prose, config-ref, README-ordering, or tool-vocabulary buckets:

- visual-companion frame template: rename CSS/HTML id #claude-content
  → #frame-content. The id is purely styling — nothing external
  references it. The brainstorm-server test that asserted the old
  string is updated in lockstep.

- visual-companion launch instructions: add a Copilot CLI section
  alongside Claude Code, Codex, and Gemini CLI; combine the Claude
  Code (macOS / Linux) and (Windows) sections so heading style
  matches the other (non-OS-qualified) platforms.

- visual-companion: "Use Write tool" → "Use your file-creation tool"
  for the cat/heredoc warning. The prohibition is what's load-
  bearing, not the tool name.

- executing-plans/SKILL.md: list all subagent-capable runtimes
  (Claude Code, Codex CLI, Codex App, Copilot CLI, Gemini CLI) and
  point at the per-platform tool refs as the source of truth.

- executing-plans/SKILL.md: relative path "using-superpowers/
  references/" → "../using-superpowers/references/" to resolve
  correctly from the executing-plans/ directory.

No bundled spec doc here — Phase D was scope-extension work that
took place across rounds, with no standalone spec authored.

tests/brainstorm-server/server.test.js

@@ -406,7 +406,7 @@ async function runTests() {
       assert(template.includes('indicator-bar'), 'Should have indicator bar');
       assert(template.includes('indicator-text'), 'Should have indicator text');
       assert(template.includes('<!-- CONTENT -->'), 'Should have content placeholder');
-      assert(template.includes('claude-content'), 'Should have content container');
+      assert(template.includes('frame-content'), 'Should have content container');
       return Promise.resolve();
     });