feat: add skill to work with branches better (#3596)

Agents seemed to have trouble finding the right calls to work with
branches (create, list, delete) and passing the right params to get it
to work. We probably don't need a big skill to get it on the right track
but a little nudge seems helpful. Doing a couple simple tasks, it saved
about half the time and tokens, so feels worthwhile. Created with the
Claude skills creator, hence the "skill.md in a bare folder"
organization - happy to move it if that's not the standard anymore.

```
Benchmark results (3 evals, with-skill vs baseline):

┌────────────────┬────────────┬────────────────────┐
│     Metric     │ With skill │   Without skill    │
├────────────────┼────────────┼────────────────────┤
│ Pass rate      │ 3/3 (100%) │ 3/3 (100%)         │
├────────────────┼────────────┼────────────────────┤
│ Avg time       │ 51s        │ 142s (2.8× slower) │
├────────────────┼────────────┼────────────────────┤
│ Avg tokens     │ 19,305     │ 36,513 (47% more)  │
├────────────────┼────────────┼────────────────────┤
│ Avg tool calls │ 5.7        │ 26 (4.5× more)     │
└────────────────┴────────────┴────────────────────┘
```

.agents/skills/lancedb-branch-ops/SKILL.md

@@ -0,0 +1,137 @@
+---
+name: lancedb-branch-ops
+description: Branch management for LanceDB tables via the REST API. Use this skill whenever someone wants to create, delete, list, or switch branches on a LanceDB table — or needs to make sure a write (metadata update, index build, etc.) lands on a specific branch instead of main. Invoke it even without the word "branch" if context makes clear they want an experimental copy of a table, want to isolate changes, or want to confirm a mutation didn't touch main. Covers: branches/list, branches/create, branches/delete, and passing "branch" in describe/update_field_metadata/create_index to target a non-main version.
+---
+
+## Goal
+
+Manage branches on a LanceDB table: list what exists, create new ones, delete stale ones, and direct read/write operations at a specific branch without touching main.
+
+## Step 0: Establish the connection
+
+Use the `lancedb-connect` skill to resolve the base URL and auth headers (`x-api-key`, `x-lancedb-database`). Skip this only if the connection is already known from the current conversation.
+
+All examples below use `{base_url}` — substitute the resolved endpoint and include the auth headers on every request.
+
+## The branch model (important)
+
+LanceDB branches are named snapshots that diverge from the table's current state at creation time. There is **no checkout command** — you never switch the whole table to a branch. Instead, you **pass `"branch": "<name>"` in the request body** of any operation to target that branch. Omitting the key (or sending an empty body) always targets main.
+
+`branches/list` returns only non-main branches. Main always exists and is not listed.
+
+## List branches
+
+```http
+POST {base_url}/v1/table/{table_id}/branches/list
+Content-Type: application/json
+
+{}
+```
+
+Response:
+```json
+{
+  "branches": {
+    "experiment-reindex": {"parentVersion": 1, "createAt": 1782506085, "manifestSize": 1029}
+  }
+}
+```
+
+If `branches` is `{}`, the table has no branches besides main.
+
+## Create a branch
+
+```http
+POST {base_url}/v1/table/{table_id}/branches/create
+Content-Type: application/json
+
+{"name": "experiment-reindex"}
+```
+
+HTTP 200 with `{}` body = success. The branch is created off the table's current state on main.
+
+Verify by calling `branches/list` and confirming the new name appears.
+
+## Delete a branch
+
+```http
+POST {base_url}/v1/table/{table_id}/branches/delete
+Content-Type: application/json
+
+{"name": "stale-2024"}
+```
+
+HTTP 200 with `{}` body = success. Only the branch pointer is removed — main and all row data remain intact.
+
+Verify by calling `branches/list` (name gone) and `describe` with no branch param (main still responds).
+
+## Operate on a specific branch
+
+Pass `"branch": "<name>"` in the body of any operation to scope it to that branch:
+
+**Read schema on a branch:**
+```http
+POST {base_url}/v1/table/{table_id}/describe
+Content-Type: application/json
+
+{"branch": "wip-branch"}
+```
+
+**Write metadata to a branch (not main):**
+```http
+POST {base_url}/v1/table/{table_id}/update_field_metadata
+Content-Type: application/json
+
+{
+  "branch": "wip-branch",
+  "updates": [
+    {
+      "path": "category",
+      "metadata": {"lancedb:description": "Product category label."},
+      "replace": false
+    }
+  ]
+}
+```
+
+**Build an index on a branch:**
+```http
+POST {base_url}/v1/table/{table_id}/create_index
+Content-Type: application/json
+
+{
+  "branch": "wip-branch",
+  "column": "category",
+  "index_type": "BTREE"
+}
+```
+
+## Verifying isolation
+
+After writing to a branch, always confirm the change did NOT land on main:
+
+```bash
+# Should show the new metadata
+curl -s -X POST {base_url}/v1/table/{table_id}/describe \
+  -H "x-api-key: <key>" -H "x-lancedb-database: <db>" \
+  -H "content-type: application/json" \
+  -d '{"branch": "wip-branch"}'
+
+# Should NOT show the new metadata
+curl -s -X POST {base_url}/v1/table/{table_id}/describe \
+  -H "x-api-key: <key>" -H "x-lancedb-database: <db>" \
+  -H "content-type: application/json" \
+  -d '{}'
+```
+
+## Quick reference
+
+| Goal | Endpoint | Body |
+|------|----------|------|
+| List all branches | `branches/list` | `{}` |
+| Create a branch | `branches/create` | `{"name": "..."}` |
+| Delete a branch | `branches/delete` | `{"name": "..."}` |
+| Read schema on branch | `describe` | `{"branch": "..."}` |
+| Write metadata on branch | `update_field_metadata` | `{"branch": "...", "updates": [...]}` |
+| Build index on branch | `create_index` | `{"branch": "...", "column": ..., "index_type": ...}` |
+| Target main (default) | any endpoint | omit `"branch"` key |