feat(builtins): add glob command (non-standard, programmatic glob matching)

[chaliy]

Summary

A non-standard builtin exposing glob matching as a command. While bash does glob expansion natively, there is no way to test if a string matches a glob pattern programmatically (without [[ ]] hacks).

Proposed Syntax

glob [OPTIONS] pattern [string...]
# or
input | glob [OPTIONS] pattern

Proposed Flags

Flag	Description
(default)	Print matching strings
-v	Invert — print non-matching
-q	Quiet — exit 0 if any match, 1 if none
-c	Count matches
--files	Match against VFS files (like find but with glob)

Use Cases

# Filter a list by pattern
echo -e "foo.rs\nbar.py\nbaz.rs" | glob "*.rs"
# foo.rs
# baz.rs

# Test if string matches pattern
if glob -q "v[0-9]*" "$tag"; then
    echo "Valid version tag"
fi

# Find files by glob
glob --files "src/**/*.rs"

# Exclude pattern
ls | glob -v "*.tmp"

llm_hint()

Some("glob: Glob matching. `echo files | glob '*.rs'` filters by pattern. `glob -q pat str` tests match. `glob --files 'src/**/*.rs'` finds files.")

[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