fix: isolate observer sessions to prevent polluting claude --resume list (#837)

Observer sessions created by claude-mem were appearing in the main Claude Code
session picker (`claude --resume`), cluttering the list with internal plugin
sessions that users never intend to resume.

In one user's case: 74 observer sessions out of ~220 total (34% noise).

## Solution

Set `CLAUDE_CONFIG_DIR` to `~/.claude-mem/observer-config/` when spawning
observer Claude processes. This stores observer session files in a separate
location, isolating them from user sessions.

## Changes

1. Added `OBSERVER_CONFIG_DIR` to paths.ts
2. Modified `createPidCapturingSpawn()` in ProcessRegistry.ts to inject
   `CLAUDE_CONFIG_DIR` environment variable

Observer sessions now write their `.jsonl` files to:
`~/.claude-mem/observer-config/projects/*/`

Instead of the user's:
`~/.claude/projects/*/`

Fixes #832

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

src/services/worker/ProcessRegistry.ts

@@ -19,6 +19,7 @@
 import { spawn, exec, ChildProcess } from 'child_process';
 import { promisify } from 'util';
 import { logger } from '../../utils/logger.js';
+import { OBSERVER_CONFIG_DIR, ensureDir } from '../../shared/paths.js';
 
 const execAsync = promisify(exec);
 
@@ -187,8 +188,15 @@ export async function reapOrphanedProcesses(activeSessionIds: Set<number>): Prom
  *
  * The SDK's spawnClaudeCodeProcess option allows us to intercept subprocess
  * creation and capture the PID before the SDK hides it.
+ *
+ * IMPORTANT (Issue #832): We set CLAUDE_CONFIG_DIR to isolate observer sessions.
+ * This prevents observer sessions from appearing in `claude --resume` list,
+ * which was causing 34%+ of resume entries to be internal plugin sessions.
  */
 export function createPidCapturingSpawn(sessionDbId: number) {
+  // Ensure observer config directory exists
+  ensureDir(OBSERVER_CONFIG_DIR);
+
   return (spawnOptions: {
     command: string;
     args: string[];
@@ -196,9 +204,15 @@ export function createPidCapturingSpawn(sessionDbId: number) {
     env?: NodeJS.ProcessEnv;
     signal?: AbortSignal;
   }) => {
+    // Inject CLAUDE_CONFIG_DIR to isolate observer sessions (Issue #832)
+    const isolatedEnv = {
+      ...spawnOptions.env,
+      CLAUDE_CONFIG_DIR: OBSERVER_CONFIG_DIR
+    };
+
     const child = spawn(spawnOptions.command, spawnOptions.args, {
       cwd: spawnOptions.cwd,
-      env: spawnOptions.env,
+      env: isolatedEnv,
       stdio: ['pipe', 'pipe', 'pipe'],
       signal: spawnOptions.signal, // CRITICAL: Pass signal for AbortController integration
       windowsHide: true