Problem

resolve_config_path() falls back to config/default.toml relative to process CWD.

• CLI / TUI: launched manually from project root — CWD is predictable, default works.
• ACP stdio / ACP HTTP: launched by an IDE (Zed, VS Code, Helix) — CWD is the user's
  workspace directory, not the Zeph project directory. config/default.toml does not
  exist there, so startup fails with a file-not-found error or silently uses defaults.

ACP users are forced to always pass --config <absolute-path> or set ZEPH_CONFIG.
No automatic fallback to a well-known location exists.

Expected behavior

All modes should be able to locate a default config without requiring an explicit flag.
Proposed resolution order extension:

1. --config <PATH>
2. ZEPH_CONFIG
3. config/default.toml (relative to CWD — preserves CLI/TUI behavior)
4. ~/.config/zeph/config.toml (XDG fallback for IDE/ACP launch context)

Acceptance Criteria

• resolve_config_path() tries ~/.config/zeph/config.toml as final fallback
• Only reached if CWD-relative path does not exist (no behavior change for CLI/TUI)
• Startup log emits a DEBUG message with the resolved path and which step matched
• Unit test covers: CLI flag, env var, CWD fallback, XDG fallback, all-missing error
• 020-config-loading/spec.md resolution order updated to reflect the new step 4

Files

• crates/zeph-core/src/bootstrap/config.rs — resolve_config_path()
• .local/specs/020-config-loading/spec.md