docs(phase3): add clangd/.clangd detail and audit .vscode/ absence

- AGENTS.md: expand LSP bullet to reference .clangd config file and
  CMAKE_EXPORT_COMPILE_COMMANDS=ON in CMakePresets.json
- AGENTS.md: add 'Editor integration' section documenting that .vscode/
  is gitignored and absent; prefer .clangd / .editorconfig / presets
- CLAUDE.md: expand Language server line to reference .clangd config
  and compile_commands.json export

AI workflow docs (docs/en/contributing/ai-workflow.md and zh equivalent)
already exist with clangd documented — no changes needed.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: LessUp

AGENTS.md

@@ -85,10 +85,14 @@ ctest --preset=ubsan
 - **AGENTS.md**: shared repo guidance for Codex-style agents and terminal copilots.
 - **CLAUDE.md**: Claude Code specific guidance.
 - **`.github/copilot-instructions.md`**: GitHub Copilot repository instructions.
-- **LSP**: prefer `clangd` backed by `compile_commands.json`.
+- **LSP**: prefer `clangd` backed by `compile_commands.json`. The `.clangd` config at the repo root points `CompilationDatabase` to `build/debug`; all CMake presets export `compile_commands.json` via `CMAKE_EXPORT_COMPILE_COMMANDS=ON`.
 - **Review model**: use `/review` before merge or after major cleanup phases.
 - **Autopilot strategy**: prefer a longer single-session implementation flow over `/fleet` unless the task is truly parallelizable.
 
+## Editor integration
+
+The `.vscode/` directory is **gitignored** and does not exist in the repository. Workspace settings live only in local developer environments. Do not commit `.vscode/` content — use the `.clangd`, `.editorconfig`, and `CMakePresets.json` files for machine-readable project conventions instead.
+
 ## Things to avoid reintroducing
 
 - HonKit / GitBook-era configuration

CLAUDE.md

@@ -21,7 +21,7 @@ Help finish this repository cleanly. Prioritize hardening, consistency, and remo
 
 - Docs stack: **VitePress**, published via GitHub Pages
 - Build system: **CMake + presets**
-- Language server: **clangd**
+- Language server: **clangd** — configured via `.clangd` at repo root (`CompilationDatabase: build/debug`); all CMake presets export `compile_commands.json` via `CMAKE_EXPORT_COMPILE_COMMANDS=ON`
 - Primary maintenance target: **Linux**
 - Repo posture: **archive-ready / low-frequency maintenance**