fix(authz): harden action composition boundaries

[tcconnally]

Summary

Post-merge hardening follow-up for the action-composition implementation delivered by PR #267 for issue #266. Issue #266 is already closed; this PR does not rely on or alter issue closure.

• reject malformed or credential-like identity references without echoing them
• bind ordinary admissions to the signed lineage authority action and verify override IDs
• reject forged prebind-only composition bindings and same-key cross-action replays
• cross-bind durable composition verdicts to prebind, scope, authority, policy, context, task, workspace, and external reference
• suppress unverified composition projections from receipts and exports
• propagate prebind through MCP and keep OpenAPI/server metadata aligned

Verification

• Full suite: 919 passed
• git diff --check and compile checks passed
• OpenAPI and MCP parity checks passed
• Candidate is based directly on merged main at 7ebeda3108807ab72a6baa3f3ce730a75074dfb7