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.

NOTICE.md

@@ -35,9 +35,19 @@ edits minimal, isolated to clearly-marked Hellion files, and reversible.
 
 Concrete example: when API 15 hit, I cherry-picked your fix for the
 BetterTTV emote regression with `git cherry-pick -x` so authorship and
-co-author trail stay intact. That is the standard I want to keep using as
-long as both projects are alive. You should never have to look at this
-fork and wonder if I quietly ate your work.
+co-author trail stay intact. That was the standard I held to as long
+as cherry-picking was viable, and you should never have to look at
+this fork and wonder if I quietly ate your work.
+
+With ChatTwo entering its rework cycle, the active cherry-pick
+pipeline is closed since v1.4.x — see [docs/UPSTREAM_SYNC.md](docs/UPSTREAM_SYNC.md)
+for the full reasoning. The attribution standard stays exactly the
+same: every existing `(cherry picked from commit ...)` line remains
+in the git history, the EUPL-1.2 anchor lines in source files are
+untouched, and this NOTICE.md remains canonical. If anything from
+this point forward originates from Chat 2 it will be a hand-port at
+most, called out as such in the commit message and source comments,
+not a `git cherry-pick`.
 
 If anything in this fork ever steps on something you would not be okay
 with, please reach out and I will fix it. Genuinely. The list of contacts
@@ -62,8 +72,10 @@ full-history-by-default position fits a much larger one, including the
 roleplaying community where chat archive is part of the play experience.
 Trying to upstream HellionChat's defaults would have meant arguing that
 Chat 2's defaults are wrong, and they are not. They are right for the
-user base ChatTwo serves. So I keep the fork separate, attribute clearly,
-and pull selected upstream patches when they apply.
+user base ChatTwo serves. So I keep the fork separate and attribute
+clearly. Active cherry-picking from upstream stopped in the v1.4.x
+cycle once Chat 2's rework made selective patches no longer portable;
+the existing cherry-pick trail stays in the git history.
 
 ## Why HellionChat left the GitHub fork network
 
@@ -72,8 +84,9 @@ that a fork is either a development branch or a dead mirror. HellionChat
 is neither. It is an independently-maintained EUPL-1.2 fork with its own
 release cadence, its own custom repo, its own user base. Detaching the
 fork-network relation just makes the situation honest. The git history,
-the cherry-pick trail, and the attribution stay exactly the same. The
-only thing that changes is the GitHub UI no longer says "forked from".
+the existing cherry-pick trail, and the attribution stay exactly the
+same. The only thing that changes is the GitHub UI no longer says
+"forked from".
 
 ## Trademarks and naming