plumb: a shell where every run is a value

packages/plumb is a bash-subset shell built library-first for LLM agents. Run cargo clippy 2>&1 | tail -n 5 and the run comes back as a value: per-stage argv, status, timing, and bounded captures of every stream, including what each pipe stage fed the next. Every stream stays addressable afterwards (${o[1][0]} is everything tail consumed, negative indexes count from the latest), so a later command can reuse any earlier stream without re-running it.

Three crates: plumb-syntax (spanned parser; unsupported bash is a loud error, never a misparse), plumb-core (cloneable Shell over shared state, tee’d capture engine with backpressure, strict semantics: pipefail, unset-var error, failglob, no word splitting), and the plumb CLI (reedline REPL with per-stage summaries and :runs / :json / :out inspection, -c, script and stdin modes, --json). A public read-only mirror is generated at indexable-inc/plumb.

The Elixir binding (plumb-ex, via unibind) exposes Plumb.Shell.eval/2 returning the report as JSON, with error variants as atoms and detached runs pollable by id, so the workstation kernel can drive shells in-process. Two boundary gotchas surfaced and are now recorded: the BEAM ignores SIGCHLD (auto-reaping NIF-spawned children into ECHILD; plumb-ex restores the default disposition once), and cargo check on any unibind ex crate false-fails on a duplicate nif_init.