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/shared/paths.ts

@@ -38,6 +38,10 @@ 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');
+
 // Claude integration paths
 export const CLAUDE_SETTINGS_PATH = join(CLAUDE_CONFIG_DIR, 'settings.json');
 export const CLAUDE_COMMANDS_DIR = join(CLAUDE_CONFIG_DIR, 'commands');