docs: close active upstream cherry-pick pipeline

Chat 2 has entered a major rework that Infi confirmed makes selective
patches no longer portable. The cherry-pick pipeline as a routine
workflow stops with the v1.4.x cycle. Documentation reflects the new
state across all touchpoints.

UPSTREAM_SYNC.md rewritten: replaces the "How I Cherry-Pick" /
"Reviewing What Is New Upstream" / "Conflict Handling" sections with
"Why Cherry-Picking Stopped", "What Closing the Pipeline Means in
Practice", "What Does Not Change", "What Could Re-Open Later".
Existing cherry-pick trails in the git history stay intact, EUPL-1.2
anchor lines and NOTICE.md remain canonical.

README.md, CONTRIBUTING.md, ROADMAP.md, THIRD_PARTY_NOTICES.md and
the PR template updated to match: cherry-pick references reframed as
historical or pointed at UPSTREAM_SYNC.md for the current state.
NOTICE.md keeps the BetterTTV cherry-pick example as a concrete past
case but adds a paragraph that the pipeline is closed and clarifies
the attribution standard is preserved unchanged.

PULL_REQUEST_TEMPLATE.md drops the "Upstream cherry-pick from Chat 2"
checkbox and the cherry-pick-path compatibility prompt. The upstream
git remote was already removed locally on 2026-05-08 (separate change,
not in this commit).

No source-file edits, no manifest version bump, no changelog entry —
this is documentation-only and ships with the next release.

.github/PULL_REQUEST_TEMPLATE.md

@@ -23,7 +23,6 @@ https://github.com/JonKazama-Hellion/HellionChat/security/advisories/new
 - [ ] Documentation only
 - [ ] Translation update
 - [ ] Build, CI or tooling change
-- [ ] Upstream cherry-pick from Chat 2
 
 ## Linked issue
 
@@ -53,7 +52,6 @@ new commands, new translations, removed behaviour. If none, write
   bump and is it covered by the existing migration tests?
 - Does this change the schema in MessageStore?
 - Does this change the repo.json or HellionChat.yaml manifest fields?
-- Does this affect the upstream cherry-pick path? See docs/UPSTREAM_SYNC.md.
 -->
 
 ## Checklist