security(deps): remove vulnerable xmas-elf dependency via embuild (fixes #399)

Removes the unused "elf" feature from the embuild dependency in
firmware/zeroclaw-esp32/Cargo.toml.

Vulnerability Details:
- Advisory: GHSA-9cc5-2pq7-hfj8
- Package: xmas-elf < 0.10.0
- Severity: Moderate (insufficient bounds checks in HashTable access)

Root Cause:
- The embuild dependency (version < 0.33) relies on xmas-elf ~0.9.1
- The "elf" feature was enabled but not actually used

Fix:
- Removed features = ["elf"] from embuild dependency
- The build.rs only uses embuild::espidf::sysenv, which doesn't require elf
- xmas-elf dependency is now completely eliminated from Cargo.lock

Verification:
- cargo build passes successfully
- grep "xmas-elf" firmware/zeroclaw-esp32/Cargo.lock confirms removal

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

firmware/zeroclaw-esp32/Cargo.toml

@@ -22,7 +22,7 @@ serde = { version = "1.0", features = ["derive"] }
 serde_json = "1.0"
 
 [build-dependencies]
-embuild = { version = "0.31", features = ["elf"] }
+embuild = "0.31"
 
 [profile.release]
 opt-level = "s"