feat: wire up ~/.config/diecut/config.toml user config file

[raiderrobert]

Problem

src/config/user.rs already defines load_user_config() and a UserConfig struct with an [abbreviations] table, but nothing in the CLI path ever calls it. The config file is effectively dead code — users can't put anything in ~/.config/diecut/config.toml and have it affect diecut behavior.

Several deferred features want persistent user preferences: git_protocol (from #131), named template bookmarks (#136), potentially others. A working config file is a prerequisite for those.

Proposed feature

• Actually invoke load_user_config() during CLI startup
• Merge user config into the resolution pipeline
• Define precedence: CLI flag > env var > config file > built-in default
• Initial keys supported:
  • git_protocol (from fix: default shortcodes to SSH and remove gh config protocol detection #131)
  • [abbreviations] (currently defined but unreachable — wire it up)
• Document location, schema, and precedence in the docs site

Acceptance criteria

• Config file loaded on every CLI invocation
• git_protocol key honored in shortcode expansion
• [abbreviations] table honored in source resolution
• Precedence order enforced and tested
• Docs section