feat(vbrief): extend vBRIEF source-of-truth to all agent-generated MD artifacts (ROADMAP.md, PRD.md)

[MScottAdams]

Problem

The vBRIEF source-of-truth principle is currently enforced only for SPECIFICATION.md. All other agent-generated MD artifacts (ROADMAP.md, PRD.md, and future structured documents) are written directly by agents without a machine-readable source file. This means:

• No schema validation on those artifacts
• No structured diff or changelog between versions
• No audit trail linking artifact content to the intent that produced it
• Direct edits to MD bypass any structural integrity checks
• #86 (artifact-branch binding) has no stable anchor -- binding requires a source file, not a rendered output

Current State

Artifact	vBRIEF Source	Enforced?
SPECIFICATION.md	vbrief/specification.vbrief.json	Yes (main.md + vbrief.md)
ROADMAP.md	None	No -- written directly
PRD.md	None	No -- written directly
Future structured docs	None	No

The vbrief/vbrief.md taxonomy currently defines exactly 5 file types (specification, plan, continue, playbook, specification-{name}). There is no slot for roadmap, PRD, or other durable structured artifacts.

Proposed Change

Extend the vBRIEF source-of-truth pattern to all durable agent-generated MD artifacts:

1. Define new vBRIEF file types in vbrief/vbrief.md taxonomy for: roadmap.vbrief.json, prd.vbrief.json, and a generic artifact-{name}.vbrief.json slot for other structured docs
2. Add render tasks (task roadmap:render, task prd:render) that generate the MD from the vBRIEF source
3. Add oplus rules to main.md and vbrief/vbrief.md prohibiting direct writes to these artifacts -- parallel to the existing SPECIFICATION.md prohibition
4. Add schema coverage for new artifact types (or reuse the existing specification.vbrief.json schema where shape matches)
5. Migrate existing ROADMAP.md and PRD.md to be generated artifacts with source files in vbrief/

Out of Scope

• CHANGELOG.md -- append-driven by convention, not generated from a structured source; exclude from this initiative
• AGENTS.md, README.md -- human-maintained, not agent-generated artifacts
• plan.vbrief.json / continue.vbrief.json -- already covered by vBRIEF taxonomy (no MD render needed)

Acceptance Criteria

• vbrief/vbrief.md taxonomy includes roadmap.vbrief.json and prd.vbrief.json (and generic artifact-{name}.vbrief.json) with lifecycle rules
• main.md contains prohibition rule: do not write ROADMAP.md or PRD.md directly -- edit the vBRIEF source and run the render task
• Render tasks exist for each new artifact type
• ROADMAP.md and PRD.md are generated from their vBRIEF sources (migration included)
• scripts/spec_validate.py or equivalent validates new source files against schema before render
• Tests cover render pipeline and direct-write prohibition
• Existing SPECIFICATION.md pipeline is unchanged

Related Issues

• Artifact-branch binding and complete audit trail for SDD #86 -- Artifact-branch binding (unblocked by this, as binding requires a source file anchor)
• More Determinism #233 -- More Determinism (same philosophy: replace probabilistic LLM writes with deterministic pipeline)
• fix: strengthen spec validation gate and rendered artifact freshness #115 -- Strengthen spec validation gate (extend validation gate to new artifact types)

[MScottAdams]

Closing as absorbed by #309 (RFC: vBRIEF-centric document model). The ROADMAP.md/PRD.md vBRIEF source-of-truth work in this issue is fully covered by RFC Story A (#310) and Story B (#311). The RFC's big-bang cutover decision (D6) supersedes a piecemeal approach.