fix(installer): mirror launch channel as default for external modules (#2321)

* fix(installer): mirror launch channel as default for external modules

When the user runs `npx bmad-method@next install`, the installer itself
runs from a prerelease, but the interactive channel gate previously hardcoded
"(all stable)" — defaulting tea/community modules to stable while bmad-method
itself was on next. The bleeding-edge launch did not flow through.

Detect the installer's own version via semver.prerelease() and default the
gate (and per-module picker) to match — "all next" for prerelease launches,
"all stable" for stable. Users keep full control: hit "n" to customize per
module, or pass explicit --channel / --pin / --next flags to override.

* fix(installer): seed channelOptions before module picker, not gate

CodeRabbit caught a label/install mismatch in the previous approach: the
module picker resolves version labels via decideChannelForModule, which runs
before _interactiveChannelGate. With channelOptions.global still null at
picker time, labels rendered from stable tags — then the gate flipped global
to 'next' and externals installed from main HEAD. Net effect on @next launches:
"tea (v1.6.0)" in the picker, but install pulled HEAD.

Move the launch detection up into promptInstall, immediately after
parseChannelOptions. Seeding channelOptions.global = 'next' before the picker
makes labels resolve from main HEAD (matching the install) and lets the
existing gate's haveFlagIntent check skip cleanly — the @next user already
declared their intent by typing it. Per-module customization remains available
via --pin / --next / --channel flags, same as for any pre-set global.