feat(help): add -h short flag for --help

[Tyagiquamar]

Change summary

Adds -h as a short flag for --help, resolving #1641. The help command/flag was previously only reachable via the long form, while most other CLI flags already have short variants.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

• Does your submission pass tests?

Changes to Core Features:

• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?

User Impact

Users can now request help with fastly -h in addition to fastly --help, matching common CLI conventions.

Are there any considerations that need to be addressed for release?

None.

[kpfleming]

Please search the codebase for every reference to --help; there are many places where the presence of that flag is evaluated and all of them would have to also be aware of the short version.

[rcaril]

Hi @Tyagiquamar - please use our PR template:

### Change summary

 <!--
Briefly describe the changes introduced in this pull request. Include context or
reasoning behind the changes, even if they seem minor. If relevant, link to any
related discussions (e.g. Slack threads, tickets, documents).
-->

All Submissions:

* [ ] Have you followed the guidelines in our Contributing document?
* [ ] Have you checked to ensure there aren't other open [Pull Requests](https://github.com/fastly/cli/pulls) for the same update/change?

<!-- You can erase any parts of this template not applicable to your Pull Request. -->

### New Feature Submissions:

* [ ] Does your submission pass tests?

### Changes to Core Features:

* [ ] Have you written new tests for your core changes, as applicable?
* [ ] Have you successfully run tests with your changes locally?

### User Impact

<!-- What is the user impact of this change? -->

### Are there any considerations that need to be addressed for release?

<!-- Any breaking changes, etc -->

[Tyagiquamar]

Updated in e031ffa (comment at pkg/app/run.go explaining the name lookup + nil guard, since kingpin defines the help flag internally).

I searched the codebase for every runtime reference to --help. The complete set of places that evaluate its presence:

• pkg/argparser/cmd.go IsHelpFlagOnly (line 288): string comparison args[0] == "--help" -> now also accepts -h.
• pkg/argparser/cmd.go IsGlobalFlagsOnly (line 340): flag table -> -h added alongside --help.
• pkg/argparser/flags.go ContextHasHelpFlag (line 334): looks up ctx.Elements.FlagMap()["help"] by clause name; with the short flag attached to the same clause, kingpin still keys the element as help, so this is unaffected (verified by the new tests).
• pkg/app/usage.go (lines 291, 344, 351, 412): all go through IsHelpFlagOnly / ContextHasHelpFlag, so they inherit the short flag support.
• pkg/app/usage.go globalFlags() map and the usage template: keyed by long name for rendering, not argument evaluation; unaffected.
• Remaining --help mentions are help-text strings (e.g. pkg/commands/compute/setup/loggers.go), remediation messages, the help command handling (separate from the flag), shell completion scripts, and Docker/goreleaser files, none of which evaluate the flag's presence.

Validation: go build ./pkg/... and go test ./pkg/argparser/ ./pkg/app/ in Docker golang:1.26-bookworm, all green (includes the new TestShortHelpFlag and TestIsGlobalFlagsOnly cases covering -h).