feat(cli): load Config in run() and inject resolvers into cmd_show

[ecalifornica]

Part of #184 (PR 2 of the plan there). Stacked on #191. path-cli only.

What

• run() loads one Config after clap parsing and passes &Config to cmd_show. No other command takes it yet.
• New providers module: one function per provider builds its PathResolver from Config::home_dir(). Call sites chain per-site overrides onto the result.
• opencode and copilot get their directory injected (with_data_dir, with_copilot_dir): their internal $XDG_DATA_HOME / $COPILOT_HOME reads win against with_home, and only an injected directory wins against the read. cursor gets $APPDATA\Cursor injected on Windows only; its resolver consults $APPDATA only there.
• The injections carry the opencode and Cursor path segments in the CLI. Proposal to move them back into the crates: Remove env reads from the provider crates #185 (comment)
• cmd_show constructs every provider via with_resolver. The pi arm chains with_sessions_dir when --base is given, so --base wins over the injected home.
• providers is compiled out on emscripten, like cmd_show. opencode and cursor are not dependencies on that target.

Behavior change

• show pi, Windows only: the CLI injects Config::home_dir(), which falls back to $USERPROFILE. The pi resolver's internal lookup reads $HOME only (pi: resolver home lookup ignores $USERPROFILE #192). With $HOME unset, show pi resolves %USERPROFILE%\.pi\agent\sessions.

Tests

• providers tests pin each resolver path to config-supplied values. Assertions stay on paths fully determined by injected values; resolver defaults that read the ambient environment are not asserted.

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