docs: define why-first vault profile (How/What/Why contract)

[maehwasoo]

Doc location

• docs/standards/vault/assistant-workflow.md
• docs/standards/vault/frontmatter-schema.md (usage guidance only; no enum changes)
• docs/standards/vault/typed-links.md
• docs/standards/vault/vault-structure.md

What’s wrong / missing?

• We want to use the Obsidian vault as a shared-memory Why layer (rationale, decisions, mental models) while keeping the repository as the What source of truth (SoT: canonical reference).
• The current vault docs define schema and ontology, but they do not clearly define a Why-first usage profile and retrieval priorities.
• Without that contract, vault curation feels high-overhead and the graph becomes noisy or stale.
• The docs also do not give a compact rule set for linking, archiving, and selecting the right layer for Why-oriented notes.

Suggested fix (optional)

• Add a “Why-first vault profile” that defines the contract: How (Codex CLI) / What (repo SoT) / Why (vault).
• Document the default retrieval policy: prefer layer: strategic|conceptual|logical and status: active|in-review; treat operational and archived as cold storage unless explicitly requested.
• Document graph hygiene guidance: keep a navigable spine via part_of (>= 1), plus a small typed-link web (1–3 non-part_of links per note).
• Add a 1-page cheat sheet for choosing layer for Why notes.
• Add an explicit default do/don’t list for linking and archiving.
• Non-goal: no tool or code changes in this issue; those should be tracked separately.