feat: Add toolchain inspect command

Summary

• Add fabrik toolchain inspect to derive a normalized toolchain contract from mise.toml.
• Include optional mise.lock metadata and a stable blake3: fingerprint for cache and remote execution plumbing.
• Support inspecting a requested mise platform with --platform, for example linux-x64, so remote worker pool provisioning can resolve platform-specific lockfile data.
• Use the workspace mise toolchain when invoking Rust tools, and build action PATH values from explicit tool directories instead of inheriting the parent shell.
• Split the core tool environment logic into selector, mise adapter, and action PATH responsibilities.
• Add coverage for human output, JSON output, lockfile data, requested platform data, missing config errors, and pinned env behavior.

Validation

• mise exec -- cargo fmt --all -- --check
• mise exec -- cargo test --workspace
• mise exec -- cargo clippy --workspace --all-targets -- -D warnings
• mise exec -- cargo build --release
• mise exec -- shellspec
• target/release/fabrik run fabrik

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