refactor(cli): remove the last environment reads below the composition root

[ecalifornica]

Last PR of the #184 series. path-cli only.

Closes #184.

What

• The free home_dir() in config.rs is deleted. Its two callers take a
  &Config already:
  • cmd_share::bail_no_sessions gains a &Config parameter and calls
    Config::home_dir.
  • share_config::resolve_remote calls Config::home_dir for the ~/
    expansion argument.
• cmd_derive::run takes a &Config and passes it to cmd_import::run. The
  p derive arm in cmd_p::run passes the &Config it already holds. This
  was the last transitional Config::load().
• Outside tests, Config::load() has one caller: run() in lib.rs.

Audit

$PATH is the only environment variable read below the composition root:
fuzzy.rs (external fzf lookup), cmd_resume.rs (harness binary lookup),
and cmd_export.rs (cursor-on-PATH check). A binary lookup is execution
environment, not configuration, so $PATH is not a Config field.
CONFIG_ENV_VARS and the Config fields are a 1:1 match, 8 for 8: APPDATA,
COPILOT_HOME, HOME, PATHBASE_URL, TOOLPATH_CONFIG_DIR,
TOOLPATH_QUERY_EXPLAIN, USERPROFILE, XDG_DATA_HOME. Every field has a
consumer: appdata, copilot_home, and xdg_data_home in providers.rs;
pathbase_url in cmd_pathbase.rs; toolpath_query_explain in cmd_query.rs;
toolpath_config_dir, home, and userprofile through Config::config_dir
and Config::home_dir.

Behavior change

• None. The same environment reaches the same code. It is read once, in
  run().

Tests

• sync::engine::tests::with_cfg builds a Config and nothing else. It drops
  the $TOOLPATH_CONFIG_DIR write and the TEST_ENV_LOCK hold, so the whole
  sync-engine suite runs without serializing against the rest of the tests.
• cmd_resume's resolve_input_url_uses_cache_on_hit_without_refetching drops
  its $TOOLPATH_CONFIG_DIR save/set/restore block and its TEST_ENV_LOCK
  hold. The injected Config pins the config directory.
• cmd_resume's resolve_input_unresolvable_errors_clearly drops its
  TEST_ENV_LOCK hold. It reads no environment.
• TEST_ENV_LOCK has two users left: the figment::Jail tests in config.rs,
  and the $PATH guard in cmd_resume. Its doc comment states this.
• tests/support/mod.rs ScopedHome keeps its $HOME and
  $TOOLPATH_CONFIG_DIR writes. The integration tests drive the CLI at its
  composition root, which reads the environment by design. Tracking: single owner for configuration #183 Phase 1 owns
  removing that machinery and adding a lint.

Follow-on

• Remove env reads from the provider crates #185 unblocks: every CLI construction site now injects, so the library
  crates' internal environment reads are dead weight.

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