v0.21.0

Highlights: Add opt-in environment controls for embedded agents, clarify malformed agent command errors, and patch denial-of-service vulnerabilities in documentation dependencies.

Changes

• Runtime/embedding: add opt-in agentProcessEnvBlock and agentProcessEnvAllow so embedders can keep their own credentials out of the environment inherited by agents, command probes and ACP terminals. Both are off by default; explicit grants and overlays still apply. Thanks @VACInc.

Fixes

• CLI: report a malformed --agent command (an unterminated quote or a quoted empty command) as a usage error with exit code 2 and acpxCode: "USAGE" instead of a runtime error. Thanks @KrasimirKralev.
• Dependencies: update the guarded filesystem runtime and development tools, and patch denial-of-service vulnerabilities in the docs Markdown renderer.