Thread PUBLIC_SITE_ORIGIN into commands.ts help-text URL

[JSONbored]

Context

Follow-up to #4613 (PR #4668), deliberately left out of
that PR's scope to keep it narrow.

src/github/commands.ts's @gittensory help command renders a command-reference section containing:

Full command reference (syntax, roles, gate boundary): ${GITTENSORY_SITE_URL}/docs/gittensory-commands

GITTENSORY_SITE_URL is imported from ./footer and used directly, unresolved against
PUBLIC_SITE_ORIGIN — the exact same self-hoster branding gap PR #4668 just fixed for
gittensoryFooter and renderRepoDocContent. A self-hoster with PUBLIC_SITE_ORIGIN configured still
gets a link to gittensory.aethereal.dev/docs/gittensory-commands in their own @gittensory help output
instead of their own domain.

Fix

Thread an env (or resolved siteUrl) parameter into the function that builds this help text, mirroring
the gittensoryFooter(env, ...) pattern PR #4668 just established, so it resolves
env.PUBLIC_SITE_ORIGIN ?? GITTENSORY_SITE_URL — consistent with the sibling maintainerControlPanelUrl
and the now-fixed gittensoryFooter in the same file. Trace the call chain from this help-text builder up
to wherever env is already in scope (likely the same commands.ts/processors.ts call sites PR #4668
already threaded).

Acceptance criteria

• The @gittensory help command's rendered command-reference link resolves
  env.PUBLIC_SITE_ORIGIN ?? GITTENSORY_SITE_URL, not the literal constant.
• Regression test covering both the PUBLIC_SITE_ORIGIN-set and unset branches.
• Small, scoped PR — this is a single-string-interpolation fix plus call-chain threading, not a
  broader refactor.

[linear-code]

JSO-3412