chore(cargo): share target-dir across repo worktrees

Author: pterror

.cargo/config.toml

@@ -1,3 +1,21 @@
+# Share one target/ dir across the main checkout and every git worktree
+# (including .claude/worktrees/* used by background agents), instead of each
+# worktree accumulating its own multi-GB target/. This relative path resolves
+# against the parent of *this* .cargo directory, i.e. the main checkout root
+# — so it's just the conventional $repo_root/target. Because
+# .claude/worktrees/* are nested inside the main checkout on disk, Cargo's
+# ancestor-directory config search finds this file (and thus this path, still
+# resolved relative to the main checkout) from any worktree too, even though
+# each worktree has its own (older) checked-out copy of .cargo/config.toml —
+# no propagation needed.
+# Cargo already mutexes concurrent builds against the same target dir via
+# target/.cargo-lock (shared lock for reads, exclusive for writes), so this
+# is safe: it just serializes builds across worktrees instead of
+# parallelizing them, in exchange for not multiplying multi-GB target/ dirs
+# per worktree. Override by setting the CARGO_TARGET_DIR env var, which
+# always takes precedence over this config file.
+[build]
+target-dir = "target"
 # Reduce target/ directory bloat
 [profile.dev]
 debug = 1  # 0=none, 1=line tables, 2=full
@@ -11,4 +29,4 @@ opt-level = 2  # Optimize deps even in dev builds
 # For faster builds with mold linker (local only), uncomment:
 # [target.x86_64-unknown-linux-gnu]
 # linker = "clang"
-# rustflags = ["-C", "link-arg=-fuse-ld=mold"]
+# rustflags = ["-C", "link-arg=-fuse-ld=mold"]