perf(rust): share one cargo target directory across checkouts

Each checkout builds its own complete target directory. Anyone keeping
several worktrees open pays it in full every time, roughly 1.8G each
after the dev profile change.

Move compiler output beside the checkout instead of inside it, so sibling
worktrees share one directory. Cargo resolves a relative target-dir
against the config file's parent, so a checkout at <path>/BrowserOS now
builds into <path>/shared-target. Measured locally: a first build takes
53.54s and produces a 1.8G shared directory; a second worktree then
builds in 20.07s and grows it only to 2.3G, reusing every dependency
artifact rather than adding another full copy.

Single-checkout clones are unaffected in practice, building one directory
higher than before.

CI is pinned back to the in-repo path with CARGO_TARGET_DIR, which takes
precedence over the config file. A workflow run has exactly one checkout
and gains nothing from sharing, and the BrowserClaw release locates its
compiled binary at packages/browseros-agent/target in three places: the
PostHog key check, the stamped version check, and packaging. Swatinem
rust-cache also resolves its cache path as workspace plus target and
never reads CARGO_TARGET_DIR, so an unpinned redirect would silently stop
caching. Both workflows that run cargo set it at the workflow level.

Anything added later that runs cargo in CI must set CARGO_TARGET_DIR too.

Author: DaniAkash

.cargo/config.toml

@@ -0,0 +1,14 @@
+[build]
+# Put compiler output beside the checkout rather than inside it, so sibling
+# git worktrees of this repo share one target directory instead of each
+# building a full private copy. Resolved relative to this file's parent, so a
+# checkout at <path>/BrowserOS builds into <path>/shared-target.
+#
+# Single-checkout clones are unaffected in practice: they simply build one
+# directory higher than before.
+#
+# CI pins CARGO_TARGET_DIR back to the in-repo path, because a workflow run has
+# exactly one checkout and gains nothing from sharing, while several release
+# steps locate the compiled binary at packages/browseros-agent/target. An env
+# var overrides this file, so anything running cargo in CI must set it.
+target-dir = "../shared-target"

.github/workflows/release-claw-server.yml

@@ -75,6 +75,13 @@ on:
 permissions:
   contents: write
 
+env:
+  # A committed .cargo/config.toml moves compiler output beside the checkout so
+  # sibling worktrees can share it. A workflow run has a single checkout and gains
+  # nothing from that, and several steps locate build output at the in-repo path,
+  # so pin it back here. An env var takes precedence over the config file.
+  CARGO_TARGET_DIR: ${{ github.workspace }}/packages/browseros-agent/target
+
 concurrency:
   # The legacy key keeps in-flight revisions of the renamed workflow serialized.
   group: release-claw-server-rust

.github/workflows/test.yml

@@ -25,6 +25,11 @@ permissions:
   contents: read
 
 env:
+  # A committed .cargo/config.toml moves compiler output beside the checkout so
+  # sibling worktrees can share it. A workflow run has a single checkout and gains
+  # nothing from that, and several steps locate build output at the in-repo path,
+  # so pin it back here. An env var takes precedence over the config file.
+  CARGO_TARGET_DIR: ${{ github.workspace }}/packages/browseros-agent/target
   BROWSEROS_APPIMAGE_URL: https://files.browseros.com/download/BrowserOS.AppImage
 
 concurrency: