Story D: Core doc updates (main.md, AGENTS.md) for vBRIEF-centric model

[MScottAdams]

Parent

Part of #309 (RFC: vBRIEF-centric document model)

Tier

Tier 2 -- Framework docs (depends on #310)

Scope

Update core framework documents to reference the new vBRIEF-centric model:

main.md:

• Replace all SPECIFICATION.md references with PROJECT-DEFINITION.vbrief.json
• Replace PROJECT.md references with PROJECT-DEFINITION.vbrief.json
• Update vBRIEF Persistence section for new folder structure and file types
• Update Decision Making section for new workflow

AGENTS.md:

• Update First Session gate to check for PROJECT-DEFINITION.vbrief.json instead of PROJECT.md + SPECIFICATION.md
• Update skill routing table with deft-directive-* names
• Update keyword triggers (refinement replaces triage)
• Update Development Process section for new file paths

Acceptance Criteria

• No references to SPECIFICATION.md as a directly-edited file remain in main.md
• No references to PROJECT.md as a standalone file remain in main.md
• AGENTS.md routing table uses deft-directive-* skill names
• AGENTS.md First Session checks for PROJECT-DEFINITION.vbrief.json
• All cross-references between main.md and AGENTS.md are consistent