Add configurable worktree root

* Add configurable worktree root

Allow new Paseo worktrees to be created under a custom worktrees.root from config.json while preserving the existing PASEO_HOME/worktrees default. Thread the resolved root through create/list/archive/ownership flows, MCP/session paths, checkout metadata, schema, docs, and targeted tests.

* Address worktree root review feedback

Use the resolved Paseo worktree base when a custom PASEO_HOME is provided, and remove an unused worktreesRoot dependency from worktree list handling.

* Fix checkout worktree path test on Windows

Use platform-native paths in the custom PASEO_HOME regression test so Windows CI compares against the same resolved root shape as production code.

* Clarify optional worktrees config docs

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Co-authored-by: Mohamed Boudra <boudra.moha@gmail.com>

docs/data-model.md

@@ -147,6 +147,9 @@ Single file, validated with `PersistedConfigSchema`.
   app: {
     baseUrl: string
   },
+  worktrees?: {
+    root?: string            // optional root for new worktrees; defaults to $PASEO_HOME/worktrees
+  },
   providers: {
     openai: { apiKey: string },
     local: { modelsDir: string }