MUL-6660 fix(issues): drop the "branch" metaphor from source-context copy (#7525)

"Branch into sub-issue" collided with git in a product that shows
`Branch: agent/<name>/<id>` on the same issue page and creates real git
branches for agent runs — the GitBranchPlus icon pushed the reading
further that way. It also described a metaphor instead of the action:
the entry creates an ordinary sub-issue anchored at one comment, with a
read-only snapshot of the thread up to it.

The menu item now reads "Create sub-issue from here", reusing the verb
the product already has for `actions.create_sub_issue`; "from here" is
unambiguous on a menu that hangs off a comment, and it is exactly what
separates this path from a plain sub-issue. The rest of the namespace
follows the same vocabulary (source comment / created from / go to
source comment), and the two comment-menu icons become MessageSquarePlus.

The summary row above a source-context sub-issue said "Branched from"
for what is structurally the same parent relation a plain sub-issue
renders as "Sub-issue of" — two names for one relation. It now reuses
that label whenever the parent is loaded, and falls back to "Created
from" for the snapshot's source issue when there is no parent to name.

zh-Hans/ja/ko already kept this word separate from git's (派生 / 分岐 /
분기 vs 分支 / ブランチ / 브랜치), so they change only to stay in one
vocabulary with the new EN wording.

Co-authored-by: J <bohan@devv.ai>
Co-authored-by: multica-agent <github@multica.ai>