[GLUTEN-10134][VL] Add ANSI analysis SKILL entry point

Co-Authored-By: Claude Opus 4 <noreply@anthropic.com>

Author: baibaichen

.github/skills/ansi-analysis/SKILL.md

@@ -0,0 +1,95 @@
+---
+name: ansi-analysis
+description: Analyze Gluten ANSI-mode test results (run dev/verify-ansi-expressions.sh, parse JSON tracker output, produce root-cause analysis and fix recommendations). Trigger on user requests like "analyze ANSI tests", "run ANSI matrix", "why is this ANSI test failing".
+---
+
+# ANSI Test Analysis Skill
+
+## Step 0 — MUST READ FIRST: shared analysis prompt
+
+Before doing anything else, read the shared prompt that defines the analysis output format and reference source locations:
+
+```
+.github/skills/ansi-analysis/shared.md
+```
+
+This file is the single source of truth — the same content is consumed by the CI Python pipeline (`.github/skills/ansi-analysis/analyze-ansi.py --ai-analysis`). Your output structure, reference source locations, and self-investigation steps MUST follow it. If the file is missing, STOP and tell the user the repo is in a broken state.
+
+## Step 1 — Decide entry point
+
+Ask the user (or infer from request):
+- Run new tests? → Step 2
+- Re-analyze existing JSON in `target/ansi-offload/`? → Step 3
+- Diagnose a single test failure? → Step 4
+
+## Step 2 — Run the verification script
+
+```bash
+./dev/verify-ansi-expressions.sh <category> <spark41|spark40|all> [--clean]
+```
+
+Categories: `cast | arithmetic | collection | datetime | math | decimal | string | aggregate | errors | all`
+
+Logs: `/tmp/ansi-matrix/latest/` (bash logs).
+JSON: `target/ansi-offload/*.json` (written by `GlutenExpressionOffloadTracker.scala`, this is the structured input for analysis).
+
+Notes from prior runs:
+- Use `all` mode in single JVM (~28 min) when full coverage is needed
+- After rebase / branch switch, run `./dev/builddep-veloxbe-inc.sh` first to refresh `libvelox.so` / `libgluten.so`
+
+## Step 3 — Analyze JSON results
+
+Two options:
+
+### 3a. Local AI orchestration (this skill, recommended for interactive review)
+
+1. Read `.github/skills/ansi-analysis/shared.md` (Step 0)
+2. List `target/ansi-offload/*.json`
+3. Read each JSON; extract: suite name, total/passed/failed/ignored counts, per-test `failCause`
+4. Apply the analysis template from shared.md verbatim (sections, tables, constraints)
+5. For each failure: extract Velox file:line from `failCause`, read those C++ files, verify root cause
+6. **Always** grep `isAnsiSupported` in `ep/build-velox/build/velox_ep/velox/functions/sparksql/specialforms/SparkCastExpr.cpp` when the failure involves Cast — most NO_EXCEPTION/Cast failures stem from the small whitelist there
+7. Output the markdown report
+
+### 3b. Python script (CI / batch)
+
+```bash
+python3 .github/skills/ansi-analysis/analyze-ansi.py \
+  --json-dir target/ansi-offload/ \
+  --ai-analysis \
+  --output ansi-report.md
+```
+
+The script loads the same shared prompt and calls the GitHub Models API.
+
+## Step 4 — Single-failure diagnosis
+
+When the user pastes one failing test:
+1. Locate its JSON entry under `target/ansi-offload/`
+2. Apply the self-investigation steps from shared.md (extract Velox file:line, check `isAnsiSupported`, cross-check `withAnsiEvalMode` in the shim)
+3. Output: Symptom / Root Cause / Fix Point / Representative Tests / Estimated Impact
+
+## Step 5 — Optional PR comment
+
+If the user wants the report posted to a PR:
+
+```bash
+gh pr comment <pr-number> --body-file ansi-report.md
+```
+
+(or use the GitHub MCP server tool when available)
+
+## Environment requirements
+
+For Step 2 (running tests):
+- `SPARK_ANSI_SQL_MODE=true`
+- `SPARK_TESTING=true`
+- `SPARK_SCALA_VERSION=2.13`
+- JVM: `-Dspark.gluten.sql.ansiFallback.enabled=false`
+- Maven profile: include `-Pdelta`
+
+## What NOT to do
+
+- Do NOT invent reference paths or line numbers — always grep / verify
+- Do NOT skip Step 0 — drift between shared.md and your output is the failure mode this skill is designed to prevent
+- Do NOT bypass the shared prompt by writing your own analysis structure