Replace the manual FUSION_CLAUDE_ACP env enable with an experimental feature
switch. `experimentalFeatures.claudeCliAcp` is ON by default (off only when
explicitly set false); the engine translates it into the FUSION_CLAUDE_ACP
dispatch the pi-claude-cli provider reads, at registerExtensionProviders time.
- Still fail-closed: with no bridge path published (acp-runtime plugin absent),
the provider falls back to `claude -p`.
- Explicit FUSION_CLAUDE_ACP env always wins (operator / test override).
- New testable helper claude-acp-enable.ts (6/6 tests); flag documented in the
core experimentalFeatures doc.
So with the acp-runtime plugin installed, Claude CLI now routes through the ACP
bridge by default; set experimentalFeatures.claudeCliAcp=false to force `-p`.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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