Add dogfood ledger artifact for shippable-demo evidence tracking

[IanFrelinger]

Summary

Creates docs/dogfood-ledger.md to track dated pass/fail evidence for Ashlar dogfood / shippable-demo truth before any autonomy or design-partner marketing claims.

Changes

• Create docs/dogfood-ledger.md with table format for dated evidence entries
• First entry dated 2026-09-05: PR P0: Close certification trust signature holes (limitations 7-9) #513 landed fail-closed verification defaults (partial close of limitations 7–9); merge commit 16125e58f098826fc1a970ce609fefe77759b5d4
• Explicit gap recorded: Limitation 7 NOT fully closed — strict production paths still lack RequireEd25519Signature (follow-up PR pending)
• Add one-line reference to the ledger in docs/DocsIndex.md trust loop section, positioned after certification-evidence.md
• Ledger is the dated evidence log; DogfoodValidation.md remains the gate catalog

Testing

• Docs-only change; no product/Forge code moves
• First entry references PR P0: Close certification trust signature holes (limitations 7-9) #513 fail-closed defaults, CertificationForgeAttackTests, and explicitly states limitation 7 remains open
• Ledger includes explicit gap notes: limitation 7 open for strict production paths; full end-to-end shippable demos still need dated entries
• Does NOT claim forged certs fully closed or that autonomy marketing is unblocked

Testing strategy (blast radius)

• Not applicable — documentation artifact only

Checklist

• make test not required (docs-only)
• Documentation created (docs/dogfood-ledger.md) and indexed (docs/DocsIndex.md)
• No TODO or NotImplementedException
• No breaking changes

Release (only when this PR ships a versioned NuGet/GHCR release)

• Not a versioned release — skip