feat(builtins): add parallel command (non-standard, GNU parallel-lite)

[chaliy]

Summary

A non-standard builtin for parallel command execution, inspired by GNU parallel and xargs -P. Even without true background & support, a dedicated parallel builtin could use tokio internally to run multiple commands concurrently.

Proposed Syntax

parallel [OPTIONS] command [:::] [args...]
# or
input | parallel [OPTIONS] command

Proposed Flags

Flag	Description
-j N	Max concurrent jobs (default: 4)
--arg {}	Placeholder for input (default: {})
-k	Keep output order (match input order)
--dry-run	Print commands without executing
--bar	Show progress to stderr
--halt now,fail=1	Stop on first failure
--halt soon,fail=1	Finish running, fail if any failed

Use Cases

# Process files in parallel
find . -name '*.json' | parallel -j 4 'jq .name {}'

# Parallel curl
echo -e "url1\nurl2\nurl3" | parallel -j 3 'curl -s {} > {}.html'

# With ::: argument list (no pipe needed)
parallel echo ::: a b c d

# Ordered output
seq 10 | parallel -k -j 4 'sleep $((RANDOM % 3)); echo {}'

# Transform files
ls *.csv | parallel -j 2 'csv to-json < {} > {.}.json'

llm_hint()

Some("parallel: Run commands concurrently. `ls *.txt | parallel -j 4 'wc -l {}'`. Like xargs -P but simpler.")

Implementation Notes

• Internally uses tokio::spawn to run interpreter instances concurrently
• Each job gets a cloned interpreter (like subshell)
• {} is replaced with the input line; {.} strips extension; {/} basename
• Output buffered per job, printed atomically (no interleaving)
• -k ensures output order matches input order
• Resource limits shared across all jobs (total commands, output size)
• This is an alternative to full background & support (feat(interpreter): real background execution with & and wait #559) — can be implemented independently

[chaliy]

Documentation Requirements

Since this is a non-standard (invented) builtin, it must be documented in two places:

1. README.md — Add to the "Built-in Commands" table under a new Extensions category row (non-standard bashkit-specific commands, clearly marked as extensions)
2. crates/bashkit/docs/extensions.md — Create or update a dedicated extensions guide covering all invented builtins, with short descriptions, examples, and rationale for why each exists

This ensures users discover these commands and understand they are bashkit-specific, not standard Bash.

[chaliy]

Resolved in PR #585 or #588