Add CLAUDE.local.md support via CLAUDE_MEM_FOLDER_USE_LOCAL_MD setting

When CLAUDE_MEM_FOLDER_USE_LOCAL_MD is set to 'true' in settings,
claude-mem writes auto-generated context to CLAUDE.local.md instead
of CLAUDE.md. This separates personal machine-generated context from
shared project instructions, aligning with Claude Code's native
CLAUDE.local.md convention where:

- CLAUDE.md = team-shared project instructions (checked into git)
- CLAUDE.local.md = personal/local context (gitignored)

Changes:
- Add CLAUDE_MEM_FOLDER_USE_LOCAL_MD setting (default: false)
- Add getTargetFilename() helper to resolve target based on settings
- Update writeClaudeMdToFolder() to accept optional target filename
- Update active-file detection to skip folders with either CLAUDE.md
  or CLAUDE.local.md being actively read/modified (issue #859 compat)
- Add 8 new tests covering filename selection, write behavior,
  content preservation, atomic writes, and active-file detection

Closes #632

src/shared/SettingsDefaultsManager.ts

@@ -49,6 +49,7 @@ export interface SettingsDefaults {
   CLAUDE_MEM_CONTEXT_SHOW_LAST_MESSAGE: string;
   CLAUDE_MEM_CONTEXT_SHOW_TERMINAL_OUTPUT: string;
   CLAUDE_MEM_FOLDER_CLAUDEMD_ENABLED: string;
+  CLAUDE_MEM_FOLDER_USE_LOCAL_MD: string;  // 'true' | 'false' - write to CLAUDE.local.md instead of CLAUDE.md
   // Process Management
   CLAUDE_MEM_MAX_CONCURRENT_AGENTS: string;  // Max concurrent Claude SDK agent subprocesses (default: 2)
   // Exclusion Settings
@@ -108,6 +109,7 @@ export class SettingsDefaultsManager {
     CLAUDE_MEM_CONTEXT_SHOW_LAST_MESSAGE: 'false',
     CLAUDE_MEM_CONTEXT_SHOW_TERMINAL_OUTPUT: 'true',
     CLAUDE_MEM_FOLDER_CLAUDEMD_ENABLED: 'false',
+    CLAUDE_MEM_FOLDER_USE_LOCAL_MD: 'false',  // When true, writes to CLAUDE.local.md instead of CLAUDE.md
     // Process Management
     CLAUDE_MEM_MAX_CONCURRENT_AGENTS: '2',  // Max concurrent Claude SDK agent subprocesses
     // Exclusion Settings