SMITH-025 Implement workspace trust and protocol hardening

[webfish0]

Outcome

Opening an untrusted repository cannot silently execute code or exhaust resources.

Scope

• Gate extensions, tasks, debug adapters, shell hooks, and project executables.
• Validate protocol frames and contribution payloads.
• Enforce message, recursion, collection, rate, and queue limits.
• Add fuzzing corpora and security response guidance.

Acceptance criteria

• T-023 and T-024 pass with no high-severity finding.

Requirements: US-009, FR-003, FR-012, FR-013, NFR-007

Source: backlog.md SMITH-025.