Creating a project whose resolved primary path already belongs to a
non-archived project now raises a clear ValueError naming the existing
project (create_project) — duplicated projects each seeded an identical
copy of the repo subtree, multiplying the duplicate-lane bug per copy.
The agent-facing project_create tool is idempotent instead: it re-activates
the existing project rather than erroring. allow_duplicate_path=True keeps
deliberate duplicates possible. Also updates the legacy non-git lane-id
expectation to the branch-style id introduced for #53329.
