docs: add vault-first Markdown roadmap

[TheZupZup]

Summary

Adds docs/obsidian-inspired-vault-roadmap.md — a design-only roadmap for evolving NexaNote toward a vault-based, Markdown-first architecture inspired by common Markdown note-taking concepts. No code changes.

The roadmap is grounded in the existing PlainMarkdownNoteStore backend, which already delivers human-readable filenames, drawings-beside-notes, deterministic ids for dropped-in .md files, and title-never-from-filename behavior.

Sections included:

• Clean-room policy — only public concepts (vault, YAML frontmatter, [[wikilinks]], #tags, backlinks); no copying of Obsidian source/UI/assets/internals.
• Goals / Non-goals
• Where we are today — what the Plain backend already provides vs. what's missing.
• Proposed vault layout (notes/, drawings/, notebooks/, private .nexanote/).
• Markdown/frontmatter format — namespaced nexanote: block, preserve-unknown-keys, drawings stay external.
• Sync implications — idempotency as the hard requirement, no duplicate imports, rebuildable index cache.
• Identity & filename rules — title precedence, stable-id precedence, promotion of derived ids to UUIDs.
• Migration path — incremental, reversible, opt-in phases A–E.
• Future features and a test plan.

Test plan

• Doc-only change — no tests required. Review for accuracy against plain_store.py / file_store.py and the clean-room constraint.

https://claude.ai/code/session_01QqiNkGZgo95NcejTs9D3bq

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