## Summary

- Add a new "Configuration Boundaries" section to the external agents
documentation explaining what settings are shared between Zed and ACP
agents
- Clarify bidirectional configuration story: what Zed forwards to agents
AND what agents inherit from native installations
- Add troubleshooting section for common user confusion points
- Update MCP docs with clearer cross-links

## Context

Users are confused about which Zed settings apply to external ACP agents
like Claude and Codex. Common questions:
- "I configured MCP in Zed but Claude can't see it"
- "My Codex config.toml isn't being used"
- "Do profiles apply to external agents?"

## Key Points

**What Zed forwards to ACP agents:**
- Model/mode selection, env vars, MCP servers (with limitations),
CLAUDE.md

**What's NOT forwarded:**
- Profiles, tool permissions settings, `.rules` files

**What agents DON'T inherit from native installations:**
- `~/.claude/` config, `~/.codex/config.toml`, native MCP servers,
authentication state

## Test plan

- [x] Review for technical accuracy against codebase
- [x] Check brand voice compliance

Release Notes:

- N/A