feat: Optional Obsidian/Markdown vault backend for persistent memories

[EyJunge1]

Problem

opencode-mem stores memories in its internal database and exposes them through its own UI.
Many users already maintain their knowledge base in an Obsidian vault (plain Markdown files).
Keeping a separate memory store creates duplicate content and makes memories harder to inspect,
edit, version, and reuse outside OpenCode.

Proposed solution

Add an optional Markdown-vault backend, compatible with Obsidian vaults.

This is an integration with an existing vault, not an attempt to recreate Obsidian or build a
second note-taking application.

The vault should remain the source of truth. opencode-mem should use its internal database
only as a rebuildable semantic-search index.

Expected behavior

• Configure a vault path, e.g. obsidianVaultPath or a generic markdownVaultPath.
• Import and index existing Markdown files from the selected folder.
• Store AI-created memories as readable Markdown files with YAML frontmatter.
• Detect file additions, edits, and deletions and update the vector index accordingly.
• Allow semantic search across indexed vault notes.
• Inject the most relevant notes into OpenCode context as memories do today.
• Preserve normal Obsidian conventions, including tags, links, folders, and user edits.
• Keep the feature optional; the current internal storage remains the default.

Example configuration

{
  "memoryBackend": "markdown-vault",
  "markdownVaultPath": "~/Documents/Obsidian/My Vault",
  "markdownMemoryFolder": "OpenCode Memories"
}