fix(runtimes): rely on AWF_HOST_PATH for Lean toolchain discovery

[Copilot]

The Lean runtime's install step couldn't be found by the agent inside AWF. The original code installed elan to $HOME/.elan and then symlinked the wrappers into /tmp/awf-tools/ on the assumption that $HOME wasn't reachable from the sandbox — but the symlinks dangle because the wrappers re-exec into $HOME/.elan/toolchains/....

AWF actually captures the host runner's PATH (plus $GITHUB_PATH entries) into AWF_HOST_PATH and re-exports it as PATH inside the chroot, so ##vso[task.prependpath] is sufficient.

Summary

• src/runtimes/lean/mod.rs — install elan to its default $HOME/.elan location and ##vso[task.prependpath] $HOME/.elan/bin. Drops the mkdir -p /tmp/awf-tools + symlink loop.
• src/runtimes/lean/extension.rs, docs/runtimes.md — note that Lean is on PATH inside the sandbox via AWF_HOST_PATH; no absolute-path guidance needed.

curl https://elan.lean-lang.org/elan-init.sh -sSf \
  | sh -s -- -y --default-toolchain stable
echo "##vso[task.prependpath]$HOME/.elan/bin"

Test plan

• cargo test (existing test_lean_prepare_steps still asserts elan-init.sh is present)
• cargo clippy --all-targets --all-features

[jamesadevine]

This was pants and not the root cause.