chore(release): prepare dstack 0.6.1

[kvinwang]

Prepares dstack 0.6.1, a security release for #1442 (guest-agent log path) and #1430 (KMS/verifier vm_config binding).

Changes

• bump the release version to 0.6.1 in every version source: dstack/Cargo.toml (and the workspace entries in Cargo.lock), os/mkosi/mkosi.conf ImageVersion, os/mkosi/versions.env DSTACK_VERSION, and the deprecated Yocto DISTRO_VERSION. The mkosi README's example output path follows
• add the 0.6.1 CHANGELOG section: security fixes, the vm_config compatibility note from fix(attest): accept one vm_config, not competing copies #1430, the hugepage vm_config memory change from feat: measure multi-node NUMA layouts of hugepage VMs #1418, and the features merged since v0.6.0

The SDKs keep their own versions and are not part of this release.

Verification

• cargo check --manifest-path dstack/Cargo.toml --workspace passes; Cargo.lock changes only workspace package versions
• ./os/mkosi/build.sh lint passes
• git diff --check is clean

After merge, the merge commit gets mkosi-os-v0.6.1, kms-v0.6.1, gateway-v0.6.1, verifier-v0.6.1 and v0.6.1, mirroring 0.6.0.