repl: :diff meta + _git_clean_cmd helper (Phase 6 commit #3)

User-driven git diff injection. The model sees the diff on the next
ask_ai turn through the existing exec_output channel.

Changes:
  - _git_clean_cmd(subcmd_and_args) helper near _scan_project_tree.
    B1: every git invocation that flows into context MUST use
    `--no-pager -c color.ui=never`. Forkpty makes git think stdout
    is a TTY, enabling both color and the pager's keypad/line-clear
    escapes — these would pollute the captured context block. The
    helper is the single chokepoint; commit #4's @<r1>..<r2> retry
    will reuse it.

  - :diff [<args>] meta:
      - Reads cwd at meta invocation (R6: differs from :tree's
        scan-time cwd capture; documented in §5).
      - Runs `_git_clean_cmd("diff " .. args)` via executor.exec.
      - Empty output -> "(no diff): <label>" status, no context append.
      - Non-zero exit -> "diff failed (exit N): <label>" status,
        no context append. git's stderr already streamed to the
        user via executor.exec's live multiplex, so the failure
        reason is visible.
      - Success -> appends "[diff <label>]\n<output>" via
        ctx:append_exec_output. Label is "(working tree)" for empty
        args, else verbatim args.
      - Status confirms injection size: "diff injected: <label> (N bytes)".

  - HELP gains :diff line with three example arg shapes; N3-resolved
    (no `staged` alias — the meta is thin pass-through to git's grammar).

Smoke verified across four scenarios in an ephemeral test repo:
  - Working-tree dirty -> 110-byte diff injected, no ANSI escapes
  - --cached -> 118-byte staged diff injected, clean
  - garbage..nonexistent -> exit 128, status + skip
  - Clean working tree -> "(no diff)", status + skip

Regression: test_safety 87/87, test_router_model 31/31, repl loads.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

repl.lua

@@ -151,6 +151,8 @@ Meta commands:
   :tree [<depth>]         scan cwd file-tree, inject as [project] block in system prompt
   :tree refresh           re-scan with last opts (or config defaults)
   :tree off               clear the [project] block
+  :diff [<git-args>]      git diff <args> -> inject as [diff ...] exec_output
+                          examples: :diff   :diff --cached   :diff main..feature
   :delegate <p> <prompt>  one-shot sub-broker call to preset <p>; prints reply
   :help                   this message
 ]]
@@ -684,6 +686,15 @@ function M.run(config)
         end
     end
 
+    -- Phase 6 (B1): every git invocation that flows back into context
+    -- MUST suppress git's interactive pager + color output. Forkpty
+    -- makes git think stdout is a TTY, which enables both. Helper
+    -- prepends `git --no-pager -c color.ui=never <subcmd_and_args>`;
+    -- used by :diff and the @<r1>..<r2> @-mention path in commit #4.
+    local function _git_clean_cmd(subcmd_and_args)
+        return "git --no-pager -c color.ui=never " .. subcmd_and_args
+    end
+
     -- Phase 6 (§6 + N4): project file-tree scanner. Prefers
     -- `git -C <dir> ls-files --cached --others --exclude-standard`
     -- when <dir> is inside a git repo (free .gitignore honor);
@@ -1763,6 +1774,29 @@ function M.run(config)
     --   :tree <N>        scan with depth=N; cached as _project_opts
     --   :tree refresh    re-scan with cached opts; else config defaults
     --   :tree off        clear ctx.project AND ctx._project_opts
+    -- Phase 6: :diff meta — `git diff <args>` (B1-clean), appends as
+    -- [diff <args>]\n<output> exec_output. Reads cwd at invocation
+    -- time (R6: differs from :tree's scan-time cwd capture). Empty
+    -- diff or git failure emits status and skips — never pollutes
+    -- context with empty or error noise.
+    meta.diff = function(args)
+        args = (args or ""):gsub("^%s+", ""):gsub("%s+$", "")
+        local cmd = _git_clean_cmd("diff " .. args)
+        local out, code = executor.exec(cmd)
+        if code ~= 0 then
+            renderer.status(("diff failed (exit %d): %s")
+                            :format(code, args == "" and "(working tree)" or args))
+            return
+        end
+        if not out or out:gsub("%s", "") == "" then
+            renderer.status(("(no diff): %s"):format(
+                args == "" and "(working tree)" or args))
+            return
+        end
+        local label = args == "" and "(working tree)" or args
+        ctx:append_exec_output(("[diff %s]\n%s"):format(label, out))
+        renderer.status(("diff injected: %s (%d bytes)"):format(label, #out))
+    end
     meta.tree = function(args)
         local sub = (args or ""):match("^%s*(%S*)") or ""
         if sub == "off" then