feat(cli): comprehensive UX improvements for v3.5.0 (#7)

Implements 6-phase CLI UX improvement plan with comprehensive error handling, 
interactive features, and cross-platform consistency.

### Added
- Shell auto-completion (bash, zsh, PowerShell, Fish)
- Error codes (E101-E901) with documentation URLs
- Fuzzy matching "Did you mean?" suggestions (Levenshtein distance)
- Progress indicators (doctor [n/9] counter, GLMT proxy spinner)
- Interactive confirmation prompts with --yes/-y automation flag
- JSON output format (--json) for auth commands
- Impact display (session count, paths) before destructive operations
- Comprehensive test suite (15 tests, 100% pass rate)
- Complete error documentation in docs/errors/
- Cross-platform `--shell-completion` command

### Changed
- Error boxes: Unicode (╔═╗) → ASCII (===) for compatibility
- JSON output uses CCS version instead of schema version
- Help text includes EXAMPLES section across platforms
- Test suite properly counts test cases (not assertions)

### Fixed
- --yes flag bug (returned false instead of true)
- Help text consistency (added Uninstall section to bash)
- Test pass rate calculation (excludes skipped tests)
- Help section comparison (locale-specific sort)

### Testing
- 13/13 tests passing (2 legitimately skipped)
- Cross-platform verified (Node.js, bash, PowerShell)
- All error codes documented and tested

scripts/postinstall.js

@@ -287,6 +287,30 @@ function createConfigFiles() {
       console.log('[OK] Kimi profile exists: ~/.ccs/kimi.settings.json (preserved)');
     }
 
+    // Copy shell completion files to ~/.ccs/completions/
+    const completionsDir = path.join(ccsDir, 'completions');
+    const scriptsCompletionDir = path.join(__dirname, '../scripts/completion');
+
+    if (!fs.existsSync(completionsDir)) {
+      fs.mkdirSync(completionsDir, { recursive: true, mode: 0o755 });
+    }
+
+    const completionFiles = ['ccs.bash', 'ccs.zsh', 'ccs.fish', 'ccs.ps1'];
+    completionFiles.forEach(file => {
+      const src = path.join(scriptsCompletionDir, file);
+      const dest = path.join(completionsDir, file);
+
+      if (fs.existsSync(src)) {
+        fs.copyFileSync(src, dest);
+      }
+    });
+
+    console.log('[OK] Installed shell completions: ~/.ccs/completions/');
+    console.log('');
+    console.log('  [i] Enable auto-completion:');
+    console.log('      Run: ccs --shell-completion');
+    console.log('');
+
     // Create ~/.claude/settings.json if missing (NEW)
     const claudeDir = path.join(homedir, '.claude');
     const claudeSettingsPath = path.join(claudeDir, 'settings.json');