feat: per-project default Claude arguments

[gi11es]

Summary

• Adds per-project default arguments for Claude Code sessions, implementing the three-tier hierarchy proposed in feat: per-session Claude Code argument customization #46 (comment): global defaults > project defaults > per-session customization
• Each tier fully replaces the one below (no merging)
• Right-click a project in the sidebar → "Default Claude Arguments…" to set project-level defaults
• Renames "Extra arguments" to "Default Claude arguments" in Settings and adds help text noting project overrides

How it works

• defaultArgs: String? on ProjectItem (runtime) and ProjectState (persistence) — nil means no override, falls back to global
• Resolution chain: perSessionDialog ?? project.defaultArgs ?? globalDefaults
• Per-session dialog pre-fills with project defaults when set, otherwise global defaults
• Setting an empty value clears the project override
• Persisted in state.json, backward-compatible (optional field decodes as nil)

Closes #46

Test plan

• Set global default args in Settings (e.g. --model sonnet), verify new Claude tabs use them
• Right-click project → "Default Claude Arguments…", set --verbose
• Create new Claude tab — should use --verbose only (project replaces global)
• Enable per-session prompt, create tab — dialog should pre-fill with --verbose
• Clear project default (empty the field, Save) — next tab should fall back to global args
• Restart Deckard — project defaults should persist

🤖 Generated with Claude Code