fix: auto-select language when doc has only one variant

[mvanhorn]

What

When a doc entry has only one language variant, resolveDocPath() now auto-selects it instead of requiring --lang. Multi-language docs still prompt for language selection as before.

Why

The MCP tool description for chub_get already documents this behavior (server.js line 57: "Auto-selected if only one"), but it was never implemented. Most docs in the registry have only one language variant, making the common case unnecessarily verbose.

Before:

chub get stripe/api --lang python   # required even though python is the only option

After:

chub get stripe/api                 # auto-selects python (the only variant)
chub get stripe/api --lang python   # still works explicitly

Fixes #49

Testing

• npm test - all 232 tests pass
• chub build content/ --validate-only - 1553 docs, 7 skills validated
• New unit test covers single-language auto-selection
• Updated e2e test to verify auto-selection works end-to-end
• Existing multi-language tests continue to pass (no behavior change for multi-lang docs)

Notes

• 3 files changed, 18 insertions, 3 deletions
• The fix is a 2-line else if branch in resolveDocPath() in cli/src/lib/registry.js
• Backward compatible - explicit --lang still works for all entries