Add timber move to relocate a worktree checkout - #117

Summary

• New timber move <name[@repo]> <path> command (alias mv) that relocates a worktree checkout via git worktree move
• Validates the destination (must not exist, must not be inside the worktree) and resolves relative paths against the cwd
• Preserves the worktree's selector name by writing the timber-name file when the destination leaves the standard managed layout, so list/switch/prune keep working under the same name; manually linked worktrees re-derive from the new basename
• Reports the relocated cwd through the path file and adds a move|mv case to the generated zsh wrapper so t move follows your shell into the new location
• Shell completion: worktree names for arg 1, directory completion for arg 2

Test plan

• go test ./internal/... — all pass, including new functional tests for relocation, name preservation, cwd path-file reporting, destination rejection, and the zsh wrapper cd behavior
• hk pre-commit hooks (constable, staticcheck, nilaway, gosec, errcheck, testifylint, gofumpt, goimports, mdlint, gitleaks, govulncheck) all pass
• Manual smoke test: created a worktree, moved it outside the worktree root, verified list --json shows the new path with the original name

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