fix(copilot): robust diff computation for cloud agent environments

The cloud agent checks out a branch like copilot/review-pr-NNN which
may not have upstream/main or origin/main refs available for merge-base.

Changes:
- Add 'main' as third fallback in merge-base resolution
- Add gh pr diff as final fallback when no merge-base is found
- Explicitly fetch origin/main in copilot-setup-steps.yml
- Applied to all 5 diff commands across both skills

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: anakrish

.github/copilot-setup-steps.yml

@@ -8,3 +8,5 @@ steps:
   - uses: actions/checkout@de0fac2e4500dabe0009e67214ff5f5447ce83dd # v6.0.2
     with:
       fetch-depth: 0 # full history needed for git diff against main
+  - run: git fetch origin main:refs/remotes/origin/main
+    name: Ensure origin/main ref is available for diff computation

.github/skills/code-review/SKILL.md

@@ -25,15 +25,20 @@ Key constraints (details in copilot-instructions.md):
 ## Step 1: Get the Diff
 
 ```bash
+# Find the merge-base with main (handles: upstream remote, origin remote,
+# local main branch, or falls back to gh pr diff for cloud agent environments).
 BASE=$(git merge-base upstream/main HEAD 2>/dev/null \
-    || git merge-base origin/main HEAD 2>/dev/null)
+    || git merge-base origin/main HEAD 2>/dev/null \
+    || git merge-base main HEAD 2>/dev/null)
 if [ -z "$BASE" ]; then
-  echo "ERROR: Cannot find upstream/main or origin/main. Cannot determine review scope."
-  exit 1
+  echo "No merge-base found. Falling back to gh pr diff."
+  gh pr diff --name-only
+  gh pr diff -- '*.rs' '*.toml' 'examples/'
+else
+  echo "Reviewing changes since: $BASE"
+  git diff "$BASE"..HEAD --stat
+  git diff "$BASE"..HEAD -- '*.rs' '*.toml' 'examples/'
 fi
-echo "Reviewing changes since: $BASE"
-git diff "$BASE"..HEAD --stat
-git diff "$BASE"..HEAD -- '*.rs' '*.toml' 'examples/'
 ```
 
 If the diff is empty, stop and report: "No changes found to review."

.github/skills/deep-review/SKILL.md

@@ -40,15 +40,20 @@ Use `read_agent` with `wait: true` to wait for each background agent.
 ## Step 1: Get the Diff and Build Inventory
 
 ```bash
+# Find the merge-base with main (handles: upstream remote, origin remote,
+# local main branch, or falls back to gh pr diff for cloud agent environments).
 BASE=$(git merge-base upstream/main HEAD 2>/dev/null \
-    || git merge-base origin/main HEAD 2>/dev/null)
+    || git merge-base origin/main HEAD 2>/dev/null \
+    || git merge-base main HEAD 2>/dev/null)
 if [ -z "$BASE" ]; then
-  echo "ERROR: Cannot find upstream/main or origin/main."
-  exit 1
+  echo "No merge-base found. Falling back to gh pr diff."
+  gh pr diff --name-only
+  gh pr diff -- '*.rs' '*.toml' 'examples/' | head -2000
+else
+  echo "Reviewing changes since: $BASE"
+  git diff "$BASE"..HEAD --stat
+  git diff "$BASE"..HEAD -- '*.rs' '*.toml' 'examples/' | head -2000
 fi
-echo "Reviewing changes since: $BASE"
-git diff "$BASE"..HEAD --stat
-git diff "$BASE"..HEAD -- '*.rs' '*.toml' 'examples/' | head -2000
 ```
 
 If the diff is empty, stop and report: "No changes found to review."
@@ -106,7 +111,9 @@ Use `model: "gpt-5.4"` in the task tool call (provides model diversity).
 > Get the diff:
 > ```
 > BASE=$(git merge-base upstream/main HEAD 2>/dev/null \
->     || git merge-base origin/main HEAD 2>/dev/null)
+>     || git merge-base origin/main HEAD 2>/dev/null \
+>     || git merge-base main HEAD 2>/dev/null)
+> # If BASE is empty, use: gh pr diff -- '*.rs' '*.toml' 'examples/'
 > git diff "$BASE"..HEAD -- '*.rs' '*.toml' 'examples/'
 > ```
 >
@@ -161,7 +168,9 @@ Use `model: "claude-opus-4.6"` in the task tool call.
 > Get the diff AND read full source files for context:
 > ```
 > BASE=$(git merge-base upstream/main HEAD 2>/dev/null \
->     || git merge-base origin/main HEAD 2>/dev/null)
+>     || git merge-base origin/main HEAD 2>/dev/null \
+>     || git merge-base main HEAD 2>/dev/null)
+> # If BASE is empty, use: gh pr diff -- '*.rs' '*.toml' 'examples/'
 > git diff "$BASE"..HEAD -- '*.rs' '*.toml' 'examples/'
 > ```
 > Then use `view` to read the full source files that were changed.
@@ -219,7 +228,9 @@ Use the default model (no `model` parameter).
 > Get the diff:
 > ```
 > BASE=$(git merge-base upstream/main HEAD 2>/dev/null \
->     || git merge-base origin/main HEAD 2>/dev/null)
+>     || git merge-base origin/main HEAD 2>/dev/null \
+>     || git merge-base main HEAD 2>/dev/null)
+> # If BASE is empty, use: gh pr diff -- '*.rs' '*.toml' 'examples/'
 > git diff "$BASE"..HEAD -- '*.rs' '*.toml' 'examples/'
 > ```
 > Use `view` to read surrounding context.
@@ -439,7 +450,9 @@ Launch **1 general-purpose agent in background mode**.
 > Get the diff:
 > ```
 > BASE=$(git merge-base upstream/main HEAD 2>/dev/null \
->     || git merge-base origin/main HEAD 2>/dev/null)
+>     || git merge-base origin/main HEAD 2>/dev/null \
+>     || git merge-base main HEAD 2>/dev/null)
+> # If BASE is empty, use: gh pr diff -- '*.rs' '*.toml' 'examples/'
 > git diff "$BASE"..HEAD -- '*.rs' '*.toml' 'examples/'
 > ```
 > Use `view` to read full source files.