Name the CLI subcommand vtctl

[rcrowley]

Vitess transitioned some time ago from the fat vtctl command to the vtctld server and its thin vtctldclient client. Never will I understand that naming decision but it has deeply bothered me for longer than I've worked here.

We hoisted vtctld up into the pscale branch vtctld subcommand and that similarly doesn't make sense - this is a client process, it shouldn't have the d suffix!

So this patch makes the subcommand canonically vtctl, adds an alias for vtctld for compatibility, and fixes up usage messages and such to follow.

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Here's a trail of bot output in case it's useful

Both names resolve — vtctl directly and vtctld as an alias (usage shows the canonical vtctl path).

Changes:

- internal/cmd/branch/vtctld/vtctld.go — Use: "vtctl <command>", added Aliases: []string{"vtctld"}, and updated Short/Long to "Run vtctl commands against a branch".
- internal/cmd/branch/vtctld/throttler.go — two pscale branch vtctld list-tablets next-step hints → vtctl.
- internal/cmd/branch/vtctld/planned_reparent.go — pscale branch vtctld planned-reparent-shard status example → vtctl.
- internal/cmd/database/throttler.go — "tablet/vtctld throttler (pscale branch vtctld throttler)" → vtctl.
- AGENTS.md — "not per-DR, not tablet/vtctld" → vtctl (matches the database throttler doc text).

Left alone (Vitess component or vtctldclient references): doc/api-client.md (vtctld API surface), internal/cmdutil/json_error.go ("active vtctld
mutation"), and all "via vtctld", "vtctld operation", "vtctld keyspaces/workflows" strings plus vtctldclient mentions in the vtctld command package and
internal/planetscale.

Build passes; tests for internal/cmd/branch/vtctld, internal/cmd/database, and internal/cmdutil pass.