Replace durable pool terminology with external throughout the codebase and documentation #105
Description
Description
Rename all references to "durable" pool mode to "external" throughout the codebase, documentation, and tests. The term "durable" implies persistence or durability when the actual meaning is that the pool discovers workers managed by an external process rather than spawning its own. "External" communicates this directly.
Motivation
The Docker integration work (#104) introduces discovery backends where the pool observes externally managed containers (Swarm services, Compose stacks). "External discovery" and "external pool" describe this pattern clearly. "Durable discovery" does not — it suggests the discovery state persists across restarts, which is not what the mode does. Aligning on "external" before adding new pool modes avoids enshrining a confusing term in more code.
Expected outcome
- All references to "durable" in pool mode context are replaced with "external" across source code, docstrings, comments, README files, and tests.
- No behavioral changes — this is a pure terminology rename.