fix(lsp): add better clangd compile_commands detection

[scarf005]

Problem

clangd sometimes reports bogus STL include errors (e.g. <string> not found) even when a project provides a CMake-generated compile_commands.json (often via a symlink).

Fix

• Make LSP root discovery search up to the git top-level (Instance.worktree), so compile_commands.json can be found even if opencode is started from a subdirectory.
• Start clangd with --query-driver (covers common compiler wrappers like ccache/sccache) so clangd can discover system include paths reliably.
• If compile_commands.json exists at the chosen root, pass --compile-commands-dir explicitly.

[github-actions]

Hey! Your PR title Fix clangd compile_commands detection doesn't follow conventional commit format.

Please update it to start with one of:

• feat: or feat(scope): new feature
• fix: or fix(scope): bug fix
• docs: or docs(scope): documentation changes
• chore: or chore(scope): maintenance tasks
• refactor: or refactor(scope): code refactoring
• test: or test(scope): adding or updating tests

Where scope is the package name (e.g., app, desktop, opencode).

See CONTRIBUTING.md for details.

[github-actions]

The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.