When the agent ran replace_in_file or write_file the user saw just
"⎿ 1 line in 12ms" and had no idea what actually changed. Now the
display shows a Claude-Code-style inline diff: an "Added N, removed M"
summary followed by colored unified-diff hunks (red minus, green plus,
cyan hunk header, dim metadata), so the change is visible at a glance.

Implementation:

- New R/diff-render.R::compute_unified_diff(old, new, path) shells out
  to `diff -u`. Returns NULL when the inputs are identical (display
  silently skips), an all-green payload for new files (old == ""), and
  a fallback summary when `diff` isn't on PATH so the tool call still
  succeeds.
- New ok_with_diff(text, diff) in R/utils.R is a thin builder; ok()
  itself is untouched so the diff-extension doesn't bleed into the
  package's general tool-result contract.
- tool_replace_in_file passes its existing `original`/`updated`
  strings; tool_write_file reads prior content before the write and
  diffs against the post-write content (taking append mode into
  account).
- turn.R threads the diff payload through outcome_text -> event;
  observer_progress() renders via render_tool_diff() when present.
- inst/bin/corteza tool_handler does the same render.
- colorize_diff() in R/cli-colors.R is the shared line painter; also
  wired into the /diff slash command so its output is colored too.
- LLM-facing result text is unchanged. The model already knows what
  it asked for; the diff is purely for the human reading the terminal.

27 new tinytests cover identical / new file / emptied / multi-hunk /
missing trailing newline / `diff`-not-on-PATH fallback.

…numbers

Codex flagged that the CLI script and corteza::chat() were making
separate decisions about color support and inline-diff layout, which
is exactly how the "CLI is colored but chat() isn't" drift just hit
the user. Centralize the shared bits before the gap widens.

Color policy is now sourced from R/cli-colors.R only:

- ansi_supported() learns NO_COLOR / FORCE_COLOR / RSTUDIO so RStudio's
  R console (which is not a tty) and override env vars both work.
- inst/bin/corteza deletes its private .ansi_supported() and inline
  16-entry color list; both surfaces call corteza:::ansi_colors().

Inline diffs rendered through the same renderer everywhere:

- render_tool_diff() now drops the redundant `--- /path` and `+++ /path`
  lines (the path is already in the tool-call title) and the `@@` hunk
  headers. It parses each hunk header for starting line numbers, walks
  the body, and emits one row per line as `NNNN +|-| content` with red
  on removals, green on additions, default on context. Truncation
  marker passes through dim.
- Tool labels: replace_in_file -> "Update", write_file -> "Write" to
  match Claude Code phrasing and match the rendered diff context.

This is the scoped step Codex suggested; a full console_ui() module
that owns every cat() in the CLI is a follow-up, filed separately.