Allow configuring a nested vault root in VS Code

[jav-ed]

Problem

In VS Code, markdown-oxide appears to use the opened workspace root as the vault root. This makes it hard to use markdown-oxide in repositories where the Markdown documentation vault intentionally lives in a nested folder.

Example repo layout:

repo/
  Project_Manag/
    Docs/
      .moxide.toml
      doc_Start.md
      Descr/
        overview.md

Inside Project_Manag/Docs/doc_Start.md, this link is correct relative to the docs vault:

[Overview](Descr/overview.md)

But when opening the full repo in VS Code, markdown-oxide reports it as unresolved, apparently because the effective vault root is repo/ instead of repo/Project_Manag/Docs.

Desired behavior

Allow the VS Code extension or server config to define a nested vault root, for example:

vault_root = "Project_Manag/Docs"

or have the VS Code extension detect the nearest .moxide.toml / .obsidian folder and start the language server with that as the root, similar to root marker behavior in other LSP clients.

Why this matters

For documentation-heavy software repos, the project root and the Markdown vault root are often not the same folder. Opening only the docs folder works around the issue, but it loses the rest of the repository context in the editor.

Current workaround

Open Project_Manag/Docs as the VS Code workspace root, or disable unresolved diagnostics. Neither is ideal.

[jav-ed]

I looked through the current source a bit and these seem like the relevant implementation points:

• The server chooses root_dir directly from LSP initialization: first InitializeParams.root_uri, then the first workspace_folders entry, then current_dir as fallback. That happens in src/main.rs around the initialize handler.
• Settings are then loaded from root_dir/.moxide.toml in src/config.rs, so .moxide.toml can tune behavior only after the root has already been chosen.
• The vault is constructed with Vault::construct_vault(&read_settings, &root_dir), so all note refnames are root-relative from that same directory.
• The VS Code extension client currently starts a single LanguageClient for all markdown files with documentSelector: [{ scheme: "file", language: "markdown" }] and no custom root selection in vscode-extension/client/src/extension.ts.

So from the current shape, this probably needs either:

1. VS Code extension-side root detection, e.g. start/route a language client using nearest .moxide.toml or .obsidian as the LSP root for a given markdown document; or
2. server-side support for a setting/initialization option like vault_root, applied before settings and vault construction are finalized.

The first option feels closer to how other LSP clients handle root markers, while the second would be useful for clients that cannot easily control root detection.