docs(agent): adopt durable-first minimal handoffs

[miso-develop]

Summary

Updates M5Authenticator's repository-defined Agent contracts to use durable-first / minimal handoffs and adds a public-repository information boundary.

Handoff model

• Handoff is a workflow ownership/state transition, not a context-transfer document.
• Material findings, evidence, decisions, implementation/test results, limitations, Human Gate state, and follow-up requirements must be persisted to an appropriate durable location before handoff.
• Chat and durable GitHub handoff records use the same canonical structural schema.
• PR-bound transitions include exact HEAD; receivers compare it with the current PR head and treat mismatches as stale handoffs requiring GitHub-state reevaluation.
• Redundant BLOCKING, narrative Finding/Evidence/Required-action/Completion-condition sections are removed from the canonical handoff.

Public repository information boundary

• Agents must determine repository visibility before writing durable GitHub content.
• Private repository names, URLs, paths, Issue/PR identifiers, branch names, internal artifact names, and other private-source metadata must not be disclosed into a public repository without explicit human approval.
• Public records should contain the public-safe resulting decision/specification/evidence, not private-source provenance.
• Private traceability or sensitive source context must remain in an approved private durable location.

Contracts updated

• .agent/AGENT_CATALOG.md
• .agent/BOOTSTRAP.md
• .agent/HANDOFF_PROTOCOL.md
• .agent/roles.yaml
• all six .agent/roles/*.md
• .agent/commands/handoff.md

No product code or product specification is changed.

Collision check

Open PR #150 changes Web source files only and does not overlap .agent/.