Add Obsidian cask + personal-repos clone bootstrap

[lukasjuhas]

Adds Obsidian and a small bootstrap that clones my personal repos on a fresh machine.

Brewfile

• New "Notes & knowledge" section with cask "obsidian" (the editor for my llm-wiki vault).

installs/repos.sh (new)

• Clones repos listed as name|owner/repo into ~/projects.
• Skips any directory that already exists, and is non-fatal if a clone fails — a brand-new machine may not have SSH/gh auth set up yet, so it warns and carries on rather than aborting setup.
• Prefers gh repo clone when gh is authed (works over HTTPS token without SSH keys), falls back to an SSH git clone.

Wiring

• Called from installs/main.sh after the OS installs, so brew + git are already present and it stays OS-agnostic. No change to setup.sh.

First entry is llm-wiki; the commented slots are there to add more repos later.