feat(builtins): add semver command (non-standard, version comparison)

[chaliy]

Summary

A non-standard builtin for semantic version parsing and comparison. Version comparison is a constant need in CI, deployment, and dependency management scripts — yet Bash has no native support, forcing awkward sort -V hacks.

Proposed Syntax

semver <subcommand> [args...]

Proposed Subcommands

Subcommand	Description
semver compare 1.2.3 2.0.0	Print -1/0/1 (like strcmp)
semver gt 1.2.3 2.0.0	Exit 0 if first > second
semver lt 1.2.3 2.0.0	Exit 0 if first < second
semver eq 1.2.3 1.2.3	Exit 0 if equal
semver gte/lte	Greater/less than or equal
semver parse 1.2.3-beta.1	Output major/minor/patch/pre/build
semver bump major|minor|patch 1.2.3	Increment component
semver valid 1.2.3	Exit 0 if valid semver
semver sort	Sort stdin versions

Use Cases

# Check minimum version
current=$(tool --version | grep -oP '\d+\.\d+\.\d+')
if semver lt "$current" "2.0.0"; then
    echo "Upgrade required"
fi

# Bump version in CI
new_version=$(semver bump patch "$(cat VERSION)")
echo "$new_version" > VERSION

# Sort release tags
git tag | semver sort

# Parse components
eval "$(semver parse 1.2.3-beta.1)"
echo "Major: $SEMVER_MAJOR"  # 1

llm_hint()

Some("semver: Version comparison. `semver gt 1.2.3 2.0.0`, `semver bump patch 1.0.0` -> 1.0.1. Subcommands: compare/gt/lt/eq/parse/bump/sort/valid.")

[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