Store branch artifacts in the semantic Tree hierarchy

[Johnny-xuan]

Summary

• project branch documents onto their accepted parent-child hierarchy instead of one flat directory
• migrate v0.1.6 projects on the first protected mutation while keeping legacy Recall and Project Map reads compatible
• move a branch and every descendant atomically during Tree Apply, including active managed worktrees and exact rollback
• route CLI, Project Map, MCP, fixtures, and packaging through one shared artifact resolver
• release the change as TreeWork 0.1.7

Upgrade behavior

New projects start with artifact layout version 2. Existing projects remain readable without mutation and migrate on the first locked write. A legacy project with an open Tree Editing Session fails closed so the unfinished candidate cannot be reinterpreted during migration.

Verification

• 72 Rust tests
• 64 Project Map UI tests and TypeScript check
• CLI migration, subtree relocation, rollback, crash recovery, and nested managed-worktree regression
• Hook, MCP, activation, macOS quarantine, plugin, and Skill validation
• production Project Map browser acceptance
• clean-package fresh/legacy/MCP/browser acceptance
• 750-branch / 1500-relation stress run and Project Map performance gate
• Clippy with warnings denied