refactor: remove GMAIL_CHANNEL_ENABLED env flag from add-gmail skill

Channel vs tool-only is now a code-level decision at skill apply time.
If the user chose channel mode, GmailChannel is wired unconditionally.
If tool-only, no channel code is added. No runtime flag needed.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.claude/skills/add-gmail/manifest.yaml

@@ -7,15 +7,12 @@ adds:
   - src/channels/gmail.test.ts
 modifies:
   - src/index.ts
-  - src/config.ts
   - src/container-runner.ts
   - container/agent-runner/src/index.ts
   - src/routing.test.ts
 structured:
   npm_dependencies:
     googleapis: "^144.0.0"
-  env_additions:
-    - GMAIL_CHANNEL_ENABLED
 conflicts: []
 depends: []
 test: "npx vitest run src/channels/gmail.test.ts"