ACP v1 requires every agent to accept text AND resource_link prompt
content; the automation rewrite dropped the resource_link half of that
baseline. Restore the old bracketed-reference flattening in the codec,
reject only beyond-baseline blocks, and update the package contract and
Agent Note.
Also release the per-session prompt slot when agent.send() throws
synchronously (an agent disposed outside the bridge would otherwise
wedge the session into permanent 'already in flight' rejections),
drop the tautological version-negotiation branch, prove the scenario
env layer reaches the snapshot subprocess, pin bridge-side fail-closed
permission errors, and correct two overpromising test names.
