Add Bash support to jido_shell via :just_bash (elixir-ai-tools/just_bash)

[mikehostetler]

Summary

Evaluate and prototype adding Bash support to jido_shell using just_bash from elixir-ai-tools/just_bash.

Linked Project

• Hex package: just_bash
• Docs: hexdocs.pm/just_bash
• Source: elixir-ai-tools/just_bash

Why Consider This

jido_shell prioritizes sandboxed, virtualized command execution. :just_bash aligns well with an in-memory execution model and secure-by-default network controls.

Tradeoffs

Pros

• Strong sandbox-oriented design (virtual filesystem, in-memory execution model).
• Better default fit for constrained/safe execution environments.
• Network access is opt-in with allowlist controls.

Cons

• Potentially narrower Bash syntax/semantic coverage than :bash.
• May require additional compatibility work for complex agent-generated scripts.
• May need adapter work to align execution/events with existing jido_shell session flow.

Decision Status

No package decision yet. This issue tracks the :just_bash option only.

Companion issue for the alternative:

• Add Bash support to jido_shell via :bash (tv-labs/bash) #24 (:bash)

Suggested Evaluation Criteria

• Syntax compatibility against real agent-generated Bash samples.
• Safety model fit with Jido.Shell.Backend architecture.
• Integration complexity with Jido.Shell.VFS and session state updates.
• Event streaming compatibility ({:output, chunk}, :command_done, cancellation).
• Performance and determinism under concurrent sessions.

[mikehostetler]

Planning issue #23 as a mergeable MVP.

Summary

• Branch: codex/issue-23-just-bash-mvp
• PR title: Implement issue #23 with JustBash-backed bash MVP
• Keep the public bash -c "<script>" and bash <file> contract unchanged.
• Make just_bash the primary interpreter for bash, with jido_shell remaining authoritative for VFS, session state, and network policy.
• Use selective fallback only when JustBash.parse/1 fails.
• Treat non-zero shell exits as normal bash completion, not framework errors.

Implementation Changes

• Add {:just_bash, "~> 0.1.0"} and introduce one internal adapter module that bridges Jido.Shell.ShellSession.State to JustBash.
• Replace direct execution in Jido.Shell.Command.Bash with: load script -> enforce existing network policy -> parse with JustBash -> execute with JustBash or fall back to the legacy sandbox on parse failure.
• Seed JustBash with current cwd, current session env, a full workspace snapshot from Jido VFS, a custom write command for compatibility, and network config derived from execution_context.network.
• After execution, compute a diff and sync back only supported persistent changes: file creates/updates/deletes, directory creates/removals, cwd, and user-visible env changes.
• Reject unsupported persistent changes with structured Jido.Shell.Error and leave the real workspace/session unchanged.
• Emit buffered stdout and stderr as {:output, chunk} events at command completion for the MVP.
• Reserve structured errors for adapter/setup failures: script load failures, policy denials, parse/setup failures, unsupported diff classes, and unexpected integration failures.
• Update docs to describe the new default runtime, parse-failure fallback, buffered output behavior, and unsupported persistent filesystem features.

Public Interfaces

• No CLI/API shape change:
  • bash -c "<script>"
  • bash <file>
  • existing session events stay the same: {:output, chunk}, {:cwd_changed, path}, :command_done, {:error, %Jido.Shell.Error{}}
• Behavior changes:
  • richer Bash syntax is accepted through just_bash
  • non-zero script exits no longer become structured framework errors
  • unsupported persistent FS features surface as structured adapter errors

Test Plan

• Keep existing bash tests passing for inline scripts, VFS script files, session cwd updates, and network allow/deny behavior.
• Add adapter tests for snapshot/diff sync, env and cwd sync filtering, parse-failure fallback, write compatibility, non-zero exits, and unsupported persistent changes.
• Add session-level coverage for :command_done on non-zero exits, buffered output emission, and unchanged policy enforcement.
• Validation target: focused bash tests during development, then full mix test before merge.

Assumptions

• Jido remains the durable source of truth for VFS, session cwd, and session env; JustBash is an execution snapshot.
• Fallback is intentionally narrow: only JustBash.parse/1 failures use the legacy dispatcher.
• The MVP supports persistence for normal file and directory edits, not full POSIX metadata fidelity.
• Buffered output at command completion is acceptable for the first mergeable PR.