[FEATURE]: Disable LSP for external directories

[guillemus]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Problem:

When agents (primary or subagents) explore external directories—such as cloned repos for reading documentation—OpenCode spawns LSP servers for those directories. This causes:

• Long startup times (30+ seconds for large TypeScript monorepos or Java/JDTLS projects)
• Wasted CPU/memory for LSP that provides no value
• No benefit—when exploring external code for context, diagnostics aren't used

Use case:

Clone a library's repo to let the LLM search its docs/source. Only need file reading—LSP is unnecessary and harmful due to startup overhead.

Proposed solution:

{
  "lsp": {
    "external_directories": false
  }
}

Current workarounds (insufficient):

• "lsp": false — Disables LSP entirely, including main project
• Per-LSP "disabled": true — Too broad, affects all directories

Related issues (not duplicates):

• #6692 — Runtime LSP toggle (addresses LLM attention drift, not external dir spawning)
• #7184 — Share LSP with IDE (reuse existing LSP, not prevent spawning)
• #6997 — LSP timeout config (doesn't prevent unnecessary spawning)