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.