feat(cli): thread Config through the document cache

[ecalifornica]

Part of #184 (PR 11 of the plan there). Stacked on #203 (branch
robert/config-resume). The cleanup PR stacks on this one. path-cli only.

What

• Every cache.rs function takes &Config as its first parameter and resolves
  the documents directory through Config::config_dir: cache_dir,
  cache_path, write_cached, cache_ref, list_cached, remove_cached.
• make_id and pathbase_cache_id are pure string functions. They are
  unchanged.
• cache.rs reads no environment variable.
• Every call site passes the &Config it already holds. Three helper
  functions gain a &Config to carry it to the cache:
  • cmd_export::load_path_doc, plus build_gemini_conversation,
    build_pi_session, and build_codex_session.
  • query::run, query::stream_files, and query::select_files.
  • sync::engine::is_unchanged.
• cmd_cache::run_ls takes the &Config that cmd_cache::run already
  receives.
• The free config_dir() in config.rs had no caller left, so it is deleted
  along with the unit test that drove it. Config::config_dir is the only
  config-directory resolution in the crate.
• p derive still loads a transitional Config and hands it to cmd_import.
  The cleanup PR owns that load.
• Emscripten: cache.rs compiles on every target, and so does the &Config
  threading in cmd_cache; only pathbase_cache_id keeps its existing
  #[cfg(not(target_os = "emscripten"))] gate.

Behavior change

• None. The cache directory is still $TOOLPATH_CONFIG_DIR/documents, falling
  back to ~/.toolpath/documents. The value is read once, in run().

Tests

• 11 of the 13 cache.rs unit tests inject a Config rooted at a tempdir. The
  helper is config_in_tempdir, which returns the Config and its TempDir.
  The tests set no environment variable and take no TEST_ENV_LOCK, so they do
  not serialize against the rest of the suite. The 2 make_id tests need no
  Config.
• query's select_files_input_only_skips_cache passes Config::default().
  That Config has no config directory, so it strengthens the assertion: an
  input-only scope that read the cache would error.
• sync::engine's with_cfg keeps its $TOOLPATH_CONFIG_DIR write. The
  cleanup PR owns it.
• The cmd_resume unit tests and the tests/resume.rs integration tests keep
  their $TOOLPATH_CONFIG_DIR scaffolding. The cleanup PR owns it.

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