ci(release): pin Bun to 1.2.21 to fix ldd failure on ubuntu-22.04

The claude sidecar (new in v0.2.0) is built with `bun build --compile`.
Bun 1.3+ produces standalone binaries that require glibc 2.36+ symbols.
The release runner is ubuntu-22.04 (glibc 2.35), so when linuxdeploy
runs ldd on the staged sidecar binary in AppDir/usr/bin/, the dynamic
loader fails on missing symbols, ldd exits 1, and linuxdeploy aborts
with std::runtime_error. Surfaced (only with --verbose) as:

  [gtk/stderr] terminate called after throwing an instance of 'std::runtime_error'
  [gtk/stderr]   what():  Failed to run ldd: exited with code 1
  ERROR: Failed to run plugin: gtk (exit code: 134)

Bun 1.2.21 targets glibc 2.31+, which the runner satisfies.

Author: Zeus-Deus

.github/workflows/release.yml

@@ -77,9 +77,19 @@ jobs:
         # build --compile`. Installed here so the stage step below can
         # produce the per-target sidecar binary before tauri-action
         # picks it up via `externalBin`.
+        #
+        # Pinned to 1.2.21 (not `latest`) because Bun 1.3+ compiles its
+        # standalone binaries against glibc 2.36+ symbols. The Linux
+        # release runner is ubuntu-22.04 (glibc 2.35), and linuxdeploy's
+        # AppImage step runs `ldd` on every executable in usr/bin/ — which
+        # invokes the dynamic loader. If the binary needs glibc symbols
+        # the loader can't resolve, ldd exits 1 and linuxdeploy aborts
+        # with `Failed to run ldd: exited with code 1`. Surfaced as the
+        # opaque "failed to run linuxdeploy" without --verbose. Bun 1.2.21
+        # targets glibc 2.31+, which the runner satisfies.
         uses: oven-sh/setup-bun@v2
         with:
-          bun-version: latest
+          bun-version: "1.2.21"
 
       - name: Stage claude-agent sidecar binary
         # Mirrors the agent-browser stage below: explicit pre-stage