chore(bots): bump engine to 3963f4b (post-#115); use REPO_ROOT not TEST_REPO_ROOT

Engine #115 (rename TEST_REPO_ROOT → REPO_ROOT, alias kept) merged as 3963f4b.
Adopt it here:
- Bump the centralized engine pin (bot-prelude engine-ref default) d9cb1e6 →
  3963f4b — moves all four bots in lockstep.
- Rename TEST_REPO_ROOT → REPO_ROOT in the engineer author (run + publish) and
  followup steps. REPO_ROOT is the clear primary name at 3963f4b; TEST_REPO_ROOT
  still works as a deprecated alias, but we standardize on the new name.

Coupled on purpose: REPO_ROOT is only read by the engine at/after #115, so the
rename and the pin bump land together. SDK/CLI pins (0.2.102 / 2.1.61) and the
flow set (bug-fix/task) are unchanged at 3963f4b.

Signed-off-by: Eric Wang <e.wang@databricks.com>
Co-authored-by: Isaac
Signed-off-by: eric-wang-1990 <e.wang@databricks.com>

Author: eric-wang-1990

.github/actions/bot-prelude/action.yml

@@ -31,7 +31,7 @@ inputs:
     # value to move every bot to a new engine commit; never @main.
     description: 'Engine commit SHA (full 40-char) to install.'
     required: false
-    default: 'd9cb1e63dc31eed550194a29d1cfd7de9c9c793b'
+    default: '3963f4be76be3b4ac0382ca0d7b6bf9d051929e6'
   engine-repo:
     description: 'owner/name of the engine repo.'
     required: false

.github/workflows/engineer-bot-followup.yml

@@ -124,11 +124,11 @@ jobs:
           DATABRICKS_TOKEN: ${{ secrets.DATABRICKS_TOKEN }}
           RUNNER_TEMP: ${{ runner.temp }}
           # The agent's working tree AND the .bot/ lookup root. run.py resolves
-          # the config at <TEST_REPO_ROOT>/.bot/config.yaml, defaulting to
-          # `internal-repo` (the driver-repo layout) when unset — which does NOT
-          # exist here (we check out at the workspace root), so this MUST be set
-          # or load_bot fails with "No such file … internal-repo/.bot/config.yaml".
-          TEST_REPO_ROOT: ${{ github.workspace }}
+          # the config at <REPO_ROOT>/.bot/config.yaml. The engine has NO path
+          # default — it fails with a clear error if this is unset — so it MUST be
+          # set (we check out at the workspace root, not the driver-repo layout's
+          # `internal-repo/`). (`TEST_REPO_ROOT` is the deprecated alias.)
+          REPO_ROOT: ${{ github.workspace }}
         run: |
           git remote set-url origin "https://x-access-token:${GH_TOKEN}@github.com/${GITHUB_REPOSITORY}.git"
           python -m databricks_bot_engine.engineer_bot.run --phase followup \

.github/workflows/engineer-bot.yml

@@ -158,7 +158,7 @@ jobs:
         # Run from RUNNER_TEMP so the engine-rendered prompt's context file
         # (issue_body.txt, resolved against cwd) is read from there and the
         # checkout stays clean — publish's leftover check fails on any untracked
-        # path in the repo. TEST_REPO_ROOT points the agent's working tree (and
+        # path in the repo. REPO_ROOT points the agent's working tree (and
         # the .bot/ lookup) at the checkout; ISSUE_NUMBER/TITLE/URL come from
         # $GITHUB_ENV (the bot config's author.env_tokens).
         #
@@ -171,7 +171,7 @@ jobs:
         env:
           MODEL_ENDPOINT: https://${{ secrets.DATABRICKS_HOST }}/serving-endpoints/databricks-claude-opus-4-8/invocations
           DATABRICKS_TOKEN: ${{ secrets.DATABRICKS_TOKEN }}
-          TEST_REPO_ROOT: ${{ github.workspace }}
+          REPO_ROOT: ${{ github.workspace }}
           RUNNER_TEMP: ${{ runner.temp }}
           FLOW: ${{ steps.ctx.outputs.flow }}
         run: |
@@ -193,7 +193,7 @@ jobs:
         env:
           GH_TOKEN: ${{ steps.prelude.outputs.token }}
           GITHUB_REPOSITORY: ${{ github.repository }}
-          TEST_REPO_ROOT: ${{ github.workspace }}
+          REPO_ROOT: ${{ github.workspace }}
           RUNNER_TEMP: ${{ runner.temp }}
           ENGINEER_BOT_APP_ID: ${{ secrets.ENGINEER_BOT_APP_ID }}
           TOUCHED_FILES: ${{ steps.author.outputs.touched_files }}