Complete command arguments from the manifest; widen the platform filter

[jasperf]

Closes tier 1 of docs/cli-ux-plan.md Phase G: G3 (argument completion) and G6 (--platform semantics). Released as 5.26.0.

G3 — argument completion from the manifest

Shell completion stopped at the command name. wp-ops db-pull <TAB> offered nothing, even though the script header declares site and env with {production|staging} — the CLI had that parsed and indexed and never put it on screen.

It now completes the argument slots too, on all three invocation forms (bare basename, category + name, full key), resolving the command the same way execution does. An ambiguous basename completes nothing: the entries behind it may declare different arguments, and it wouldn't run either.

What a slot offers depends on what the manifest knows about it:

Slot	Offers
@arg with {a|b} choices	those values, described
site / site-name	site names from the detected Trellis project's group_vars/*/wordpress_sites.yml
a path-shaped name (input, *-file, *-dir, …)	hands back to the shell's own file completion
anything else	one line of ActiveHelp: name, required/optional, description
a token starting with -	the command's @flag names, plus --help and --where

Three decisions worth flagging for review:

• A bracketed manifest value is never offered as a completion. {example.com}, {~/wp-cli.phar}, {/opt/plesk/php/8.2/bin/php} are placeholders showing the shape of an answer, not defaults. Inserting one would be worse than offering nothing, so they appear as "e.g." inside the hint.
• Site names are read from the project silently. resolveTrellisDir() confirms a detected directory interactively, which a completion function must never do, so completionSites() takes $TRELLIS_DIR or a silently detected project. internal/detect/sites.go reads it with a line scanner rather than pulling a YAML dependency into the binary for four lines of well-known structure — a stopgap until M5's shared site registry exists.
• Flags are skipped, not parsed, when counting slots. wp-ops re-parses no script's flag grammar (DisableFlagParsing everywhere), so it cannot know whether the token after --host is that flag's value or the next positional. Counting only non-flag tokens is right for the common cases and, in the --flag value positional case, offers the previous slot — wrong in a way that costs a keystroke, not a mistake, since nothing is inserted without the user picking it.

Two assertions in dispatch_test.go pinned the old two-token grammar ("no completions once a basename is already chosen"). They now pin the unresolvable-name case, with a comment recording that the reversal is deliberate.

G6 — --platform trellis includes the commands that need no WordPress

FilterByPlatform matched @platform exactly, so --platform trellis returned the 29 Trellis commands and hid the 32 tagged any — the image converters, git helpers and release scripts that run perfectly well on a Trellis box. The flag reads as "what can I run here?", and that answer was wrong by 32 commands. It now returns 61; --platform wordpress returns 51.

The widening stops there. Rolling wordpress into a trellis filter would return the whole catalog, at which point the flag says nothing. --platform any stays exact, since "what needs no WordPress at all" is a real question and that is the only way left to ask it — which is why the separate --platform-only the plan floated wasn't needed.

catalog.RunsOnPlatform holds the relation in one place, shared with list.go's per-category filterEntriesByPlatform so the category view and the summary counts can't disagree. search keeps its [platform] badge under --platform now: a filtered result set mixes [trellis] rows with [any] ones, so the badge still distinguishes them. The listing scope under trellis ops widens with it, via defaultPlatform().

Verification

go vet and go test ./... green. Beyond the unit tests, completion was driven through a real interactive zsh over a pty against the installed _wp-ops script:

• wp-ops db-pull <TAB> → the project's site names, with the argument's description
• wp-ops db-pull example.com <TAB> → production staging
• wp-ops jpg-to-webp <TAB> → the hint above an ordinary file menu

The installed completion script already supports ActiveHelp, so no reinstall is needed to see the hints.