Add Config at the composition root

[ecalifornica]

Part of #183. path-cli only. No library changes. No user-visible changes.

Work

• Add a Config struct. Extract it one time in run() with figment:
  • Serialized::defaults, then Env.
  • It owns TOOLPATH_CONFIG_DIR, HOME, the harness root variables (XDG_DATA_HOME, COPILOT_HOME, APPDATA), PATHBASE_URL, TOOLPATH_QUERY_EXPLAIN.
  • The env layer emits values as verbatim strings (a small provider wrapping Env). No per-field deserializers.
• Change config.rs::config_dir() to read from Config, not from env.
• Change the ~30 provider construction sites to the injection functions that exist today:
  • Convo::new() becomes Convo::with_resolver(...).
  • derive_* wrappers become derive_*_with(...).
  • Commands get a &Config parameter. Command structure does not change.
• For opencode and copilot, inject the directory (with_data_dir, with_copilot_dir), not the home.
  • Their env reads still exist and win against with_home.
  • The provider issue deletes those reads.
• Test the env-to-Config mapping with figment::Jail.

PR plan

• PR 1: Config, figment load, config_dir().
• Then one PR per command module for its construction sites. The first one adds the run() extraction and passes &Config down.
• Old and new construction can coexist until the provider issue lands.

[ecalifornica]

One note: cmd_share.rs has its own private home_dir() helper (near line 639), used to tilde-shorten display paths. It is not a provider construction site, so it can survive the ~30-site sweep and only get caught later by #187's lint. Read the home from Config instead and delete the helper.