Replace 'none' project with 'personal' default project

[bborn]

Summary

• Eliminates the 'none' project concept where tasks had no project assignment
• Every task now requires a project, with 'personal' as the default
• The 'personal' project has a dedicated git-backed worktree directory for artifacts

Changes

• Database Migration: Automatically creates 'personal' project on startup, migrates existing tasks with empty project
• Git Repository: Initializes ~/.local/share/task/personal/ as a git repo with main branch
• Task Creation: Defaults to 'personal' project when none specified
• UI Form: Removes empty project option, defaults to 'personal' (can be overridden by working directory detection)
• Executor: Enforces project requirement, no longer creates arbitrary per-task directories
• Tests: Added TestPersonalProjectCreation to verify functionality

Test plan

• All database tests pass including new TestPersonalProjectCreation test
• All CLI tests pass
• Personal project directory is created and initialized as git repo
• Existing tasks with empty project are migrated to 'personal'
• New tasks default to 'personal' project
• Binary builds successfully

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