Group top-level CLI commands by user intent

[phinze]

A prospect flagged that miren --help was hard to navigate as a deployer. The top-level was a flat list of 30+ commands with server-administration sitting right next to deploy and rollback. For someone who only ever deploys an app, that's a lot of noise.

First instinct was to dim operator commands so they faded into the background, but that felt user-hostile, and it didn't solve the navigation problem so much as paper over it. Pivoted to positive grouping: five named buckets organized by what the user is trying to do (Getting started, Monitoring your app, Configuring your app, Client operations, Server operations). Group headers render in bold yellow (matching our table-header convention) with the existing faint sub-command counts. The exercise was itself useful in noticing where the CLI surface had drifted (a couple of follow-ups noted but not landed here, like download release and the debug outliers).

The rendering side moved into the runtime layer. mflags' role is to parse arguments and own the command tree; this PR makes the runtime render its own top-level help (and intercepts miren, miren -h, and miren help to all go through the same path). The data side landed in the companion mirendev/mflags#14, where we added a CommandGroupProvider interface, a Group field on ChildEntry and CommandDoc, and exported GetDirectChildren so callers can walk the tree. Same boundary makes future help-rendering polish a runtime concern, not a mflags one.

Along the way: miren help <space-separated path> now works the same as the dotted form, the help footer mentions both miren help <cmd> and miren <cmd> --help, and the help alias standalone doc page was retired (aliases content already lives in docs/docs/app-toml.md#aliases). A TestAllTopLevelCommandsHaveKnownGroup test catches new top-level commands that forget to declare a group, so the grouping stays complete as the CLI grows.

Closes MIR-845

[coderabbitai]

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• docs/command-sidebar.json
• docs/docs/command/alias.md
• docs/docs/command/disk.md
• docs/docs/command/help-alias.md
• docs/docs/command/help.md
• docs/docs/commands.md

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📝 Walkthrough

Walkthrough

This change introduces a help grouping system for the CLI. Help groups organize top-level commands into labeled categories for the top-level help output. The PR defines group constants, adds infrastructure to tag commands and sections with groups, implements grouped help rendering, wires it into the CLI entry point, tags all existing commands, improves help command parsing, and adds validation that all commands have assigned groups.

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_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[phinze]

New help in action:

[evanphx]

my.. it's so... beautiful.