Docs: roadmap for provenance + trust tiers (trust-aware packing)

[phenomenoner]

Adds concrete roadmap items to make context packing “trust-aware” (inspired by recent security write-ups about skill registries becoming an attack surface).

Key additions:

• Principle: trust-aware by design (untrusted by default unless explicitly promoted)
• New epic: Provenance + trust tiers (schema + promotion/quarantine policy)
• Context Packer acceptance criteria updated to require trust gating + provenance hints
• Thought link: 1Password post used to motivate the above

Docs-only.