fix: respect CLAUDE_CONFIG_DIR for plugin paths (#626)

[Kuroakira]

Problem

When users set CLAUDE_CONFIG_DIR to a custom location (e.g., ~/.claude-work), multiple claude-mem features fail because paths were hardcoded to ~/.claude.

The most visible failure is in HealthMonitor.ts (reported in #626):

ENOENT: no such file or directory, open '/Users/$USER/.claude/plugins/marketplaces/thedotmack/package.json'

Root Cause

Several files independently calculated the plugin path using path.join(homedir(), '.claude', ...) instead of respecting CLAUDE_CONFIG_DIR. This is a long-standing issue - PR #226 attempted a partial fix but was deferred, and it didn't include HealthMonitor.ts.

Solution

1. Added MARKETPLACE_ROOT constant to src/shared/paths.ts that respects CLAUDE_CONFIG_DIR
2. Updated all files with hardcoded paths to use centralized constants from paths.ts

Files Changed

File	Change
src/shared/paths.ts	Added MARKETPLACE_ROOT constant
src/services/infrastructure/HealthMonitor.ts	Use MARKETPLACE_ROOT for version check
src/shared/worker-utils.ts	Import MARKETPLACE_ROOT instead of local definition
src/services/worker/BranchManager.ts	Import MARKETPLACE_ROOT instead of local definition
src/services/integrations/CursorHooksInstaller.ts	Use MARKETPLACE_ROOT and CLAUDE_CONFIG_DIR
src/services/context/ObservationCompiler.ts	Use CLAUDE_CONFIG_DIR for transcript path

Testing

• Build passes
• All hardcoded ~/.claude references replaced with environment-aware constants
• Backwards compatible: defaults to ~/.claude when CLAUDE_CONFIG_DIR is not set

Fixes #626