Support CLAUDE_CONFIG_DIR and CODEX_HOME

[ecalifornica]

Related to #183. Blocked on #184, so the new reads are born in Config.

• Claude Code supports CLAUDE_CONFIG_DIR. Codex CLI supports CODEX_HOME. We read neither.
• Result: for users who relocated their harness data, we find no sessions. No error is shown; the sessions are silently absent.
• We honor COPILOT_HOME and XDG_DATA_HOME already. This makes the set consistent.
• The reads go into path-cli's Config, not into the provider crates.