XDG migration

[nfgrep]

Closes #247

This moves:

• .roast/initializers -> $WORKFLOW_DIR/initializers
  • We could also put them in ~/.config/roast/initializers, but thus far the examples I've seen have all had initializers at a "workflow" level, not in ~.
• .roast/cache -> $XDG_CACHE_HOME/roast/function_calls (default ~/.cache/roast/function_calls).
• .roast/sessions -> $XDG_DATA_HOME/roast/sessions (default ~/.local/share/roast/sessions).
• .roast/sessions.rb -> $XDG_DATA_HOME/roast/sessions.db (default ~/.local/share/roast/sessions.db).

Users are asked to run roast xdg-migrate in the case where they still have these legacy paths:

% exe/roast execute examples/api_workflow/workflow.yml
🔥🔥🔥 Everyone loves a good roast 🔥🔥🔥

[2025-07-11 22:29:27] WARN: ⚠️  DEPRECATION WARNING:
Found legacy .roast directory at /Users/nathanfaber-good/src/github.com/Shopify/roast@xdg-not-dot-roast/.roast that should be migrated to XDG directories:
From: /Users/nathanfaber-good/src/github.com/Shopify/roast@xdg-not-dot-roast/.roast/initializers
To: /Users/nathanfaber-good/src/github.com/Shopify/roast@xdg-not-dot-roast/examples/api_workflow/initializers

Please run: roast xdg-migrate /Users/nathanfaber-good/src/github.com/Shopify/roast@xdg-not-dot-roast/examples/api_workflow to migrate your data

When running roast xdg-migrate they're taking through pickers asking if they'd like to migrate each dir and if they'd like to delete it after its migrated.

After a deprecation period (say 1-3mo), we delete the XDGMigrate class and call XDG the new default.