feat(state): Alirkan mention_index lewat ToolCtx dan AppStateRest, bangun index di background thread

Menambahkan field mention_index: MentionIndex ke ToolCtx/ToolCtxBuilder dan
AppStateRest, mengikuti pola dir_cache yang sudah ada. AppStateRest::new()
sekarang men-spawn thread background yang men-walk setiap workspace root
via ignore::Walk, membangun daftar path file (dengan prefix [N] untuk root
selain yang pertama) dan mengisi mention_index — data ini yang akan dipakai
fitur autocomplete @file-mention.

src/tool/mod.rs

@@ -47,6 +47,7 @@ pub struct ToolCtx {
     pub memory_dir: PathBuf,
     pub worktrees_dir: PathBuf,
     pub dir_cache: std::sync::Arc<tokio::sync::RwLock<super::app::state::misc::DirCache>>,
+    pub mention_index: super::app::state::misc::MentionIndex,
     pub origin: crate::app::state::types::Origin,
     pub graduated_checks: Vec<GraduatedCheck>,
     pub lsp_manager: Arc<Mutex<crate::app::lsp::LspManager>>,
@@ -94,6 +95,7 @@ pub struct ToolCtxBuilder {
     pub memory_dir: PathBuf,
     pub worktrees_dir: PathBuf,
     pub dir_cache: std::sync::Arc<tokio::sync::RwLock<super::app::state::misc::DirCache>>,
+    pub mention_index: super::app::state::misc::MentionIndex,
     pub origin: crate::app::state::types::Origin,
     pub graduated_checks: Vec<GraduatedCheck>,
     pub lsp_manager: Arc<Mutex<crate::app::lsp::LspManager>>,
@@ -110,6 +112,7 @@ impl Default for ToolCtxBuilder {
             memory_dir: PathBuf::new(),
             worktrees_dir: PathBuf::new(),
             dir_cache: std::sync::Arc::new(tokio::sync::RwLock::new(super::app::state::misc::DirCache::new())),
+            mention_index: super::app::state::misc::MentionIndex::new(),
             origin: crate::app::state::types::Origin::Main,
             graduated_checks: Vec::new(),
             lsp_manager: Arc::new(Mutex::new(crate::app::lsp::LspManager::new())),
@@ -138,6 +141,7 @@ impl ToolCtxBuilder {
             memory_dir: self.memory_dir,
             worktrees_dir: self.worktrees_dir,
             dir_cache: self.dir_cache,
+            mention_index: self.mention_index,
             origin: self.origin,
             graduated_checks: self.graduated_checks,
             lsp_manager: self.lsp_manager,