🤖 ci: pin bun version to 1.3.5 (#1283)

Prevents lockfile churn from different bun versions reformatting
`bun.lock`.

**Changes:**
- CI: `setup-mux` action now uses `1.2.15` instead of `latest`  
- Local: `.tool-versions` for asdf/mise users

---
_Generated with `mux` • Model: `anthropic:claude-opus-4-5` • Thinking:
`high`_

Author: ammar-agent

.github/actions/setup-mux/action.yml

@@ -6,7 +6,7 @@ runs:
     - name: Setup Bun
       uses: oven-sh/setup-bun@v2
       with:
-        bun-version: latest
+        bun-version: 1.3.5
 
     - name: Get Bun version
       id: bun-version

.tool-versions

@@ -0,0 +1 @@
+bun 1.3.5