feat: build a read-only execution handoff

[dapi]

What changed

• add handoff build for deterministic Markdown or JSON execution handoffs
• collect explicit Memory Bank documents, frontmatter dependencies, local links, document evidence sections, Git commits/changed files, and caller-supplied verification reports
• keep declared priming context separate from observed execution context
• report unresolved sources and reject output paths that would overwrite evidence
• add Feature Flow fixtures and CLI/package tests
• document the command and architecture boundary

Why

Memory Bank needs a compact, reproducible, read-only projection for resuming one concrete task without semantic inference or raw session data.

Verification

• go test -count=1 -race ./...
• go vet ./...
• git diff --check
• manual Markdown and JSON fixture runs

Refs #47