Refactor filesystem security and execution controls across BashSession

[zac]

Summary

• extract the shell language internals in BashSession into focused control-flow, expansion, and syntax helper files
• move filesystem primitives onto the shared Workspace package and update session/filesystem APIs to use reexported FileSystem types
• add scoped RunOptions, execution limits, permission handling, and tighter network-policy enforcement across shell commands, git, and embedded Python
• expand secret handling to provider-backed references, durable keychain support, and stricter redaction/resolution flows
• refresh tests and trim documentation to match the new API surface and behavior

Testing

• swift test