fix: simple ESLint errors — regex, unused vars, unused expressions

- eslint.config.js: ignore coverage/ directory (generated files)
- frontmatter.ts: no-regex-spaces — use {2} instead of literal spaces
- wikilinks.ts: no-useless-escape — remove unnecessary \[ escape
- ai-chat.ts: remove unused _model param from getContextLimit
- noteListHelpers.ts: remove unused _modifiedFiles param from filterEntries
- NoteList.tsx: replace ternary expression statement with if/else
- useTabManagement.ts: remove stale eslint-disable-next-line comment

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/utils/noteListHelpers.ts

@@ -1,4 +1,4 @@
-import type { VaultEntry, SidebarSelection, ModifiedFile } from '../types'
+import type { VaultEntry, SidebarSelection } from '../types'
 
 export interface RelationshipGroup {
   label: string
@@ -196,6 +196,6 @@ function filterByFilterType(entries: VaultEntry[], filter: string): VaultEntry[]
   return []
 }
 
-export function filterEntries(entries: VaultEntry[], selection: SidebarSelection, _modifiedFiles?: ModifiedFile[]): VaultEntry[] {
+export function filterEntries(entries: VaultEntry[], selection: SidebarSelection): VaultEntry[] {
   return filterByKind(entries, selection)
 }