fix(github): thread PUBLIC_SITE_ORIGIN into the @gittensory help command reference link

[JSONbored]

What

Completes the self-hoster branding sweep #4668 (#4613) started. The @gittensory help command's
rendered command-reference section linked to a hardcoded GITTENSORY_SITE_URL/docs/gittensory-commands
— the same branding gap #4668 fixed for gittensoryFooter and renderRepoDocContent, just discovered
after that PR shipped and deliberately left out of its scope to keep it small. A self-hoster with
PUBLIC_SITE_ORIGIN configured still got a link to gittensory.aethereal.dev in their own
@gittensory help output instead of their own domain.

Fix

Threads env through commandSections → helpSections (the only command that renders this link),
mirroring the gittensoryFooter(env, ...) pattern from #4668. Extracted a small commandReferenceUrl(env)
helper using new URL(path, origin) — the same idiom footer.ts's maintainerControlPanelUrl already
uses — rather than naive string concatenation, since a PUBLIC_SITE_ORIGIN with a trailing slash would
otherwise produce a double slash (caught by my own first draft of the regression test failing).

Testing

2 new tests: the link follows PUBLIC_SITE_ORIGIN (including the trailing-slash case), and
commandReferenceUrl falls back to the default site if PUBLIC_SITE_ORIGIN is malformed. Existing
helpSections/buildPublicAgentCommandComment tests updated for the new required env param. 100%
coverage on every changed line/branch. Typecheck clean.

Fixes #4670

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.14%. Comparing base (8ad748b) to head (43a296f).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4684   +/-   ##
=======================================
  Coverage   94.14%   94.14%           
=======================================
  Files         436      436           
  Lines       38526    38530    +4     
  Branches    14048    14049    +1     
=======================================
+ Hits        36269    36273    +4     
  Misses       1599     1599           
  Partials      658      658           

Files with missing lines	Coverage Δ
src/github/commands.ts	98.17% <100.00%> (+0.01%)	⬆️

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