fix(setup): harden global omp symlink against bun pm -g bin failure

The root `setup` script used `ln -sfn ... "$(bun pm -g bin)/omp"`. On
fresh hosts where `$HOME/.bun/install/global/package.json` is missing
`bun pm -g bin` aborts with "No package.json was found", the command
substitution expands to empty, and `ln` tries to write `/omp` —
failing with `Permission denied` (or, on a writable filesystem,
silently linking under `/`).

Move the logic into `scripts/link-omp.sh`:

- Capture `bun pm -g bin` and ignore its failure.
- Fall back to `${BUN_INSTALL:-$HOME/.bun}/bin`, matching Bun's own
  install convention.
- `mkdir -p` the target so the link works even when nothing has yet
  populated the global bin dir.

Fixes #3701

package.json

@@ -94,7 +94,7 @@
   },
   "overrides": {},
   "scripts": {
-    "setup": "bun install && bun run build:native && bun --cwd=packages/coding-agent link && ln -sfn \"$(pwd)/packages/coding-agent/scripts/omp\" \"$(bun pm -g bin)/omp\"",
+    "setup": "bun install && bun run build:native && bun --cwd=packages/coding-agent link && sh scripts/link-omp.sh",
     "dev": "bun --cwd=packages/coding-agent src/cli.ts",
     "dev:timing": "PI_TIMING=x bun --cwd=packages/coding-agent --preload ../utils/src/module-timer.ts src/cli.ts",
     "stats": "bun --cwd=packages/coding-agent src/cli.ts stats",

scripts/link-omp.sh

@@ -0,0 +1,31 @@
+#!/bin/sh
+# Install the dev `omp` wrapper into Bun's global bin directory.
+#
+# Replaces the bun-shebang symlink that `bun --cwd=packages/coding-agent link`
+# creates (pointing at `src/cli.ts`) with the safer wrapper at
+# `packages/coding-agent/scripts/omp`. See that wrapper's header comment for the
+# bunfig.toml-preload bug it works around.
+#
+# We resolve Bun's global bin path defensively because `bun pm -g bin` aborts
+# (`No package.json was found for directory "$HOME/.bun/install/global"`) on
+# fresh hosts where the global install has not been initialized. Falling
+# through that error would expand `$(bun pm -g bin)/omp` to `/omp` and try to
+# write under `/` — see https://github.com/can1357/oh-my-pi/issues/3701.
+set -e
+
+repo_root=$(CDPATH= cd -- "$(dirname -- "$0")/.." && pwd -P)
+target=$repo_root/packages/coding-agent/scripts/omp
+
+if [ ! -x "$target" ]; then
+	echo "link-omp: target wrapper not found or not executable: $target" >&2
+	exit 1
+fi
+
+global_bin=$(bun pm -g bin 2>/dev/null || true)
+if [ -z "$global_bin" ]; then
+	global_bin=${BUN_INSTALL:-$HOME/.bun}/bin
+fi
+
+mkdir -p "$global_bin"
+ln -sfn "$target" "$global_bin/omp"
+echo "link-omp: linked $global_bin/omp -> $target"