…dError (#3483)

Follow-up to #3474: converts the remaining raw-output
`bail!("{stderr}")` sites to typed `CommandError`, so every
failed-command display goes through the one normalizing constructor
(`\r` stripping, canonical `✗ <command> failed (exit N)` header + output
gutter).

**Sites converted:**

- `step/shared.rs` (`git ls-files` in ignored-entry listing) and
`step/commit.rs` (`git add` into the temp index) — the two
commands-layer git sites.
- The five claude/codex plugin CLI calls in
`commands/config/{plugins,codex}.rs`, collapsed into one private
`run_plugin_cli` helper in `commands/config/mod.rs`. Their failure
snapshots now show the canonical header + stderr gutter instead of `✗
claude plugin marketplace add failed: <stderr>`.
- `execute_llm_command` in `llm.rs` — its hand-rolled
stderr/stdout/exit-code fallback ladder was `from_failed_output`'s job;
deleted in favor of the constructor. The `LlmCommandFailed` wrappers
read the detail via `ErrorExt::display_message()`, which extracts the
captured output for command failures and matches the old text in the
common cases; the picker summary pane guards on the error type
(`format_summary_error`) — command failures surface their captured
output, everything else keeps the full anyhow chain the pane always
showed (folded in from review). The shell program is recorded by
basename only (same rule as `render_llm_invocation`), so a no-output
failure doesn't leak the Git Bash install path on Windows.

**Deliberately left:** the two `gh pr list` / `glab mr list` bails in
`picker/prs.rs`. Their consumer is the skim-footer warning sink where
the current one-line text is the intended display, and picker text
sanitization was explicitly dropped (#3177).

**Behavior note** (intended, but visible): an LLM command that emits on
both streams before failing now shows both in the error block
(`combined_output()` joins stderr + stdout); the old ladder showed
stderr only.

**Testing:** three new error-path unit tests (`list_ignored_entries` /
`stage_to_temp_index` via corrupt-config and bad-pathspec,
`execute_llm_command` exit-code + basename assertion); the five plugin
sites are covered by the existing failing-mock integration tests whose
snapshots this updates; `test_generate_summary_llm_error` covers the
summary-pane command-failure arm and
`format_summary_error_keeps_chain_for_non_command_errors` the
chain-preserving arm.

> _This was written by Claude Code on behalf of max_

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

)

Step-12 drift fold for the v0.68.0 release: #3483 (the follow-up
completing #3474) merged during the release PR CI window, so it ships in
0.68.0 but the changelog entry only covered the eight #3474 sites. The
entry now covers the full cutover and cites #3483.

> _This was written by Claude Code on behalf of max_