P1: Implement per-tool/per-workflow AuthZ for HTTP transport

[Neftedollar]

Parent epic: #236

Problem

Current HTTP auth model is coarse-grained and does not express fine tool-level permissions.

Scope

• Add policy layer mapping caller identity/token -> allowed workflows/tools/actions.
• Enforce authorization in tool dispatch path.
• Return explicit FORBIDDEN-class structured errors.

Acceptance Criteria

• Different tokens can be scoped to different workflow/tool sets.
• Unauthorized calls fail fast before execution.
• Policy is testable and documented with examples.

Non-goals

• External IAM provider integrations in first pass.