rename lean4checker input to leanchecker

[kim-em]

This PR renames the lean4checker action input to leanchecker, while keeping lean4checker as a deprecated alias for backward compatibility.

It also updates the implementation to prefer the bundled leanchecker binary when the active Lean toolchain ships it, and otherwise fall back to the external lean4checker repository for older toolchains.

Details

• add a new leanchecker input in action.yml
• keep lean4checker as a deprecated alias
• rename docs and workflow text to use leanchecker
• replace the external-checker-only script with run_leanchecker.sh
• detect bundled support via elan which leanchecker plus an executable check
• fall back to cloning/building lean4checker when the bundled binary is unavailable
• add functional-test coverage for both:
  • legacy fallback on leanprover/lean4:v4.26.0
  • bundled leanchecker on leanprover/lean4:v4.28.0

Compatibility note

Lean gained the bundled leanchecker binary in nightly-2026-01-09, which is included in v4.28.0-rc1 and later. Older toolchains still need the external fallback.