docs: use pinned Foundry for semver-lock

[Mabolla]

Summary

• replace the foundryup recovery step for semver-lock failures with the repository-pinned mise toolchain
• run semver-lock through mise exec so local tooling matches the version pinned in .mise.toml

Why

foundryup can install a Foundry version that differs from the repository's pinned toolchain, which can leave contributors out of sync with CI and reproducibility-sensitive outputs such as semver-lock hashes and snapshots.

Closes #403.

Validation

• git diff --check
• verified the change is limited to README.md

Note: mise is not installed in the local Windows environment used for this documentation-only change, so the documented command itself was not executed locally.

[cb-heimdall]

🟡 Heimdall Review Status

Requirement	Status	More Info
Reviews	🟡 0/2	Denominator calculation Show calculation 1 if user is bot 0 1 if user is external 0 2 if repo is sensitive 0 From .codeflow.yml 1 Additional review requirements Show calculation Max 0 0 From CODEOWNERS 0 Global minimum 0 Max 1 1 1 if commit is unverified 1 Sum 2