fix: use cwd instead of CLAUDE_CONFIG_DIR for observer session isolation (#845)

The previous approach (PR #837) set CLAUDE_CONFIG_DIR to isolate observer
sessions from `claude --resume`. However, this broke authentication because
Claude Code stores credentials in the config directory.

This fix uses the SDK's `cwd` option instead:
- Observer sessions run with cwd=~/.claude-mem/observer-sessions/
- Project name = path.basename(cwd) = "observer-sessions"
- Sessions won't appear when running `claude --resume` from actual projects
- Authentication works because ~/.claude/ config is preserved

Changes:
- ProcessRegistry.ts: Remove CLAUDE_CONFIG_DIR override from spawn
- SDKAgent.ts: Add cwd option to query() pointing to observer dir
- paths.ts: Rename OBSERVER_CONFIG_DIR to OBSERVER_SESSIONS_DIR

Fixes regression from #837

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

src/shared/paths.ts

@@ -38,9 +38,9 @@ export const USER_SETTINGS_PATH = join(DATA_DIR, 'settings.json');
 export const DB_PATH = join(DATA_DIR, 'claude-mem.db');
 export const VECTOR_DB_DIR = join(DATA_DIR, 'vector-db');
 
-// Isolated config directory for observer sessions (Issue #832)
-// This prevents observer sessions from appearing in `claude --resume` list
-export const OBSERVER_CONFIG_DIR = join(DATA_DIR, 'observer-config');
+// Observer sessions directory - used as cwd for SDK queries
+// Sessions here won't appear in user's `claude --resume` for their actual projects
+export const OBSERVER_SESSIONS_DIR = join(DATA_DIR, 'observer-sessions');
 
 // Claude integration paths
 export const CLAUDE_SETTINGS_PATH = join(CLAUDE_CONFIG_DIR, 'settings.json');