feat(module): add btop/bmon/iftop/iotop/nmon/powertop monitoring modules

[ycpss91255]

Summary

Adds six apt-archetype optional monitoring modules for the small-tools
modularization program. Each installs its like-named apt package via
module_use_apt_archetype, keeps detect() / is_recommended()
module-defined, and overrides doctor() with a real health probe that
verifies the tool actually runs.

Module	apt pkg	Purpose	doctor probe
btop	btop	Resource monitor (modern top)	btop --version
bmon	bmon	Bandwidth monitor	bmon -V
iftop	iftop	Network bandwidth by host	command -v iftop
iotop	iotop	I/O monitor	command -v iotop
nmon	nmon	Performance monitor	command -v nmon
powertop	powertop	Power consumption monitor	command -v powertop

iftop / iotop / nmon / powertop ship no clean version flag, so their
doctor probe verifies the binary resolves on PATH.

Details

• Category optional, tag monitoring, SUPPORTED_UBUNTU=("22.04" "24.04" "26.04"),
  RISK_LEVEL=low, VERSION_PROVIDED=apt-managed.
• Each declares an en + zh-TW DESCRIPTION and a TEST_VERIFY_CMD matching
  its doctor probe.
• All satisfy the 10-function module contract (ADR-0002), enforced by
  test/unit/module/contract_conformance_spec.bats (test(module): module-iterating contract conformance meta-test #305), and are both
  standalone- and engine-invocable.
• doc/module/INDEX.md regenerated via ./script/gen-module-index.sh
  (54 modules).
• [Unreleased] CHANGELOG entry added.

Files

• module/{btop,bmon,iftop,iotop,nmon,powertop}.module.sh
• test/unit/module/{btop,bmon,iftop,iotop,nmon,powertop}_spec.bats
• doc/module/INDEX.md, doc/changelog/CHANGELOG.md

Test plan

Ran all gates blocking-foreground in Docker via justfile.ci:

• just -f justfile.ci test-unit — green (4214+ tests, incl. contract_conformance 5/5)
• just -f justfile.ci test-integration — green (24 tests)
• just -f justfile.ci lint — green (ShellCheck / fish syntax / Hadolint)

Each new module spec covers: metadata sanity, is_installed false on a clean
container, dry-run no-ops, idempotency, sidecar lifecycle (ADR-0001),
standalone CLI, and verify / doctor including a real-binary probe.