refactor(cli): self-register commands via ServiceLoader

[rlaope]

Summary

ArgusCli.java held a 67-line manual registration list. Every command had to be imported and added to a LinkedHashMap by hand, and the ordering of that list happened to drive help output.

This PR moves discovery to a META-INF/services/io.argus.cli.command.Command manifest. ArgusCli loads commands via ServiceLoader and sorts within each CommandGroup alphabetically for the help screen — so behaviour no longer depends on registration order.

TuiCommand stays explicitly registered after the loader pass because it takes the populated commands map as a constructor argument and so cannot satisfy ServiceLoader's no-arg-constructor requirement.

• ArgusCli.java: −149 lines (66 manual register calls + 67 imports gone)
• META-INF/services/io.argus.cli.command.Command: +66 lines (one FQN per command)
• 17-command smoke pass on the fatJar; help still shows "67 commands"

Why this and not full SPI unification

The earlier architectural sketch suggested merging CLI Command with the server-side DiagnosticCommand SPI. That overlap exists on metadata (name/group/description) but the execute(...) shapes diverge — CLI commands carry ANSI / TUI / --format=json / CliConfig concerns that CommandContext doesn't model, and only ~29 of the 67 CLI commands have a server sibling. A full unification would touch ~80 files for a mostly-cosmetic win. The actual friction was the central manual registration list, which this PR eliminates.

Behaviour changes

• Help output is now alphabetical within each CommandGroup. Previous order was a hand-curated list that drifted as commands were added at the end. The group order itself (the CommandGroup enum's declaration order) is unchanged.
• Adding a CLI command no longer requires editing ArgusCli.java — drop the class and add one line to the META-INF file.

Test plan

• ./gradlew build -x integrationTest passes
• argus --help shows "67 commands" line
• Spot-checked 17 commands resolve via ServiceLoader (alert/ps/gc/heap/threads/doctor/harness/zgc/gcwhy/nmt/jfr/profile/flame/top/trace/tui/watch)
• Verify help output sort order on the user side meets expectations