fix(launcher): resolve gateway startup issue in tui mode

[darrenzeng2025]

Fixes #1704

Problem

The picoclaw-launcher-tui was not respecting PICOCLAW_HOME and PICOCLAW_CONFIG environment variables when determining the config file path. This caused a configuration mismatch between the TUI and the gateway command when these environment variables were set.

Root Cause

• TUI launcher used hardcoded ~/.picoclaw/config.json path
• Gateway command supported PICOCLAW_HOME and PICOCLAW_CONFIG environment variables
• When users set these environment variables, TUI saved config to one location but gateway read from another

Solution

• Updated ConfigPath() to check PICOCLAW_CONFIG environment variable first
• Updated ConfigDir() to check PICOCLAW_HOME environment variable first
• Added comprehensive unit tests for both functions
• Added documentation comments explaining the priority order

Priority Order (consistent with gateway command)

1. PICOCLAW_CONFIG - direct config file path
2. PICOCLAW_HOME - custom config directory
3. Default: ~/.picoclaw/config.json

Testing

• Added unit tests covering all environment variable combinations
• Verified priority order works correctly
• Tests pass on both Unix and Windows platforms

Files Changed

• cmd/picoclaw-launcher-tui/internal/config/store.go - Fixed config path resolution
• cmd/picoclaw-launcher-tui/internal/config/store_test.go - Added unit tests