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/services/worker/SDKAgent.ts

@@ -16,7 +16,7 @@ import { SessionManager } from './SessionManager.js';
 import { logger } from '../../utils/logger.js';
 import { buildInitPrompt, buildObservationPrompt, buildSummaryPrompt, buildContinuationPrompt } from '../../sdk/prompts.js';
 import { SettingsDefaultsManager } from '../../shared/SettingsDefaultsManager.js';
-import { USER_SETTINGS_PATH } from '../../shared/paths.js';
+import { USER_SETTINGS_PATH, OBSERVER_SESSIONS_DIR, ensureDir } from '../../shared/paths.js';
 import type { ActiveSession, SDKUserMessage } from '../worker-types.js';
 import { ModeManager } from '../domain/ModeManager.js';
 import { processAgentResponse, type WorkerRef } from './agents/index.js';
@@ -101,10 +101,15 @@ export class SDKAgent {
     // Run Agent SDK query loop
     // Only resume if we have a captured memory session ID
     // Use custom spawn to capture PIDs for zombie process cleanup (Issue #737)
+    // Use dedicated cwd to isolate observer sessions from user's `claude --resume` list
+    ensureDir(OBSERVER_SESSIONS_DIR);
     const queryResult = query({
       prompt: messageGenerator,
       options: {
         model: modelId,
+        // Isolate observer sessions - they'll appear under project "observer-sessions"
+        // instead of polluting user's actual project resume lists
+        cwd: OBSERVER_SESSIONS_DIR,
         // Only resume if BOTH: (1) we have a memorySessionId AND (2) this isn't the first prompt
         // On worker restart, memorySessionId may exist from a previous SDK session but we
         // need to start fresh since the SDK context was lost