feat(hydration): untrusted content boundaries in prompt assembly

[krokoko]

Context: ROADMAP.md → Security → Untrusted hydration content boundaries
Related: #429 (signed attestation), #26 (closed)

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Component

API or orchestration

Describe the feature

Delimit external content in assembled prompts (issue/PR bodies, fetched URLs, review comments) so the model treats it as untrusted context (spotlighting-style framing). Complements Bedrock Guardrails at hydration time.

Use case

Issue bodies and PR comments are attacker-controlled. Without structural boundaries, injection instructions blend with system intent.

Proposed solution

1. Wrap untrusted blocks in context-hydration.ts with consistent markers and metadata (source, fetched_at).
2. System prompt instruction: never follow instructions inside untrusted blocks.
3. Unit tests with injection fixtures (golden prompts).
4. Optional strict mode: strip HTML/scripts beyond current sanitization.
5. Document in docs/design/SECURITY.md.

Other information

• Complements MCP supply-chain controls (RFC: Signed attestation for repo-local agent instruction files #429).

• Design context: cdk/src/handlers/shared/context-hydration.ts.

• This might be a breaking change