[build] document Bazel worktree output-base cleanup

AGENTS.md

@@ -38,8 +38,12 @@ Shared/high-risk areas:
 - `bazel-*/` treat as generated output
 
 ### Agent workspace
-The `.local/` directory (gitignored) is available for generated artifacts or temporary files:
-- Use `--output_base=.local/bazel-out` if bazel output directory restricted
+The `.local/` directory (gitignored) is available for generated artifacts or temporary files.
+- When building in a git worktree, put Bazel's output base inside the worktree so it is reclaimed when
+  the worktree is removed: add `startup --output_base=.local/bazel-out` to the worktree's
+  `.bazelrc.local`, or pass `--output_base=.local/bazel-out`. By default Bazel keys the output base on
+  the checkout path and never deletes it when the worktree is removed, leaking several GB per discarded
+  worktree. See "Using Worktrees" in README.md for the shared cache setup.
 
 ## Cross-binding consistency checks
 When changing user-visible behavior, compare with at least one other binding:

README.md

@@ -154,6 +154,25 @@ dependencies (e.g. `http_archive` tarballs). Both directories grow unbounded ove
 periodically if disk space matters. Keep the cache on the same filesystem as your checkouts so Bazel
 can hardlink instead of copy.
 
+Bazel also creates a separate **output base** (compiled outputs, analysis cache, and the Bazel
+server) per checkout path. Unlike the caches above, it is **not** removed when you delete a worktree —
+so frequently created and discarded worktrees leak gigabytes of stale output.
+
+On macOS/Linux, make a worktree self-cleaning by pointing its output base inside the worktree. Add
+this to that worktree's `.bazelrc.local`:
+
+```
+startup --output_base=.local/bazel-out
+```
+
+`.local/` is gitignored and excluded in `.bazelignore`, so removing the worktree removes its output
+base with it. The shared `--disk_cache`/`--repository_cache` above still keep downloads and action
+outputs shared across worktrees.
+
+(Windows users should instead keep `startup --output_user_root=C:/tmp` in `.bazelrc.windows.local` as
+described above, to avoid path-length limits — do not nest the output base deeper inside the repo on
+Windows.)
+
 ## Building
 
 Selenium is built using a common build tool called [Bazel](https://bazel.build/), to