release: Agent Platform 0.6.8

[b10x-bot]

Summary

• release the principal-owned agent isolation boundary as 0.6.8
• refresh the deterministic OpenAPI checksum for the package version
• record the governed component-only promotion plan

Validation

• cargo fmt --all --check
• cargo clippy --workspace --all-targets --locked -- -D warnings
• cargo test --workspace --locked
• aep artifact validate --strict