v0.7.21 Credential request contract

Credential request contract for borrowed agents and plugins.\n\n- Adds value-free credential request metadata to the public local credential map: provider, env name, allowed hosts, allowed operations, scope, setup URL, input mode, save target, and broker mode.\n- Clarifies that host-bound broker isolation requires a real local process/IPC boundary; legacy runtime env injection is explicitly not equivalent.\n- Updates auto-activation, source-of-truth, runtime boundary, schema, and project memory templates for Desktop/terminal secure credential prompts and future broker enforcement.\n\nVerification: scripts/verify-package.sh, scripts/public_safety_check.sh, architecture sync-check.