fix(path): enforce filepath.Clean sanitization on cache paths and invocation directories

[HarshalPatel1972]

Description

This PR introduces explicit filepath.Clean() sanitization fences around user-supplied execution directory flags and runtime environment cache parameters. This prevents trailing slashes or redundant separators from propagating anomalies downstream into file operations.

Technical Context

When an execution invocation sets a working directory or reads the MAGEFILE_CACHE environment variables, the raw strings can infiltrate the pipeline with malformed layouts (e.g., /tmp/mage//bin or trailing delimiters).

While filepath.Join handles cleaning internally under specific criteria, direct usage of raw path references during isolated file lookups or short-circuit evaluations bypasses this safety net, causing native file system operations to error out with cryptic no such file or directory diagnostics.

Resolution Strategy

• Cache Directory Normalization: Intercepted the boundary return in mg/runtime.go:CacheDir() to guarantee a sanitized, consistent path configuration across all cache lookups.
• Invocation Sanitization: Enforced an early filepath.Clean() wrapper on the incoming inv.Dir state during early-stage flag processing in mage/main.go, cleaning up execution directory formats before child modules evaluate targets.

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