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.

CONTRIBUTING.md

@@ -15,9 +15,11 @@ to make a contribution land smoothly.
 - Read the [README](README.md) so you understand the scope: a
   privacy-focused, EUPL-1.2-licensed Dalamud plugin that intentionally
   removes the upstream webinterface and ships privacy-first defaults.
-- Read [`docs/UPSTREAM_SYNC.md`](docs/UPSTREAM_SYNC.md). Cherry-picks
-  from upstream Chat 2 are selective and deliberate; not everything
-  that lands there belongs here.
+- Read [`docs/UPSTREAM_SYNC.md`](docs/UPSTREAM_SYNC.md). Active
+  cherry-picking from upstream Chat 2 has ended in the v1.4.x cycle;
+  HellionChat continues as an independent codebase. Existing
+  upstream-derived code keeps its attribution. New contributions
+  stand on their own and do not need to be cherry-pick-compatible.
 - Read [`SECURITY.md`](SECURITY.md). Anything security-sensitive goes
   through a private advisory, never a public issue or PR.
 - Read the [Code of Conduct](CODE_OF_CONDUCT.md).
@@ -43,9 +45,11 @@ to make a contribution land smoothly.
   "Was gegenüber Chat 2 fehlt".
 - Features that bypass the privacy filter or weaken the default
   retention behaviour without an explicit, documented opt-in.
-- Sweeping refactors that touch large parts of the codebase. They make
-  selective upstream cherry-picks much harder and the maintenance cost
-  outweighs the benefit for a one-person project.
+- Sweeping refactors that touch large parts of the codebase. The
+  maintenance cost outweighs the benefit for a one-person project.
+  (This used to be doubly important because of the upstream
+  cherry-pick path; that path is closed now, but the rule still
+  holds on its own merits.)
 - AI-generated code dropped in without disclosure or human review. See
   [`docs/AI_DISCLOSURE.md`](docs/AI_DISCLOSURE.md) for how I handle
   AI assistance on my side; I expect comparable transparency from
@@ -117,9 +121,15 @@ Hellion-specific strings live in
 direct pull requests.
 
 The upstream Chat 2 strings in `HellionChat/Resources/Language.*.resx`
-are **not** translated here. They are owned by the upstream project
-and synced in via cherry-pick. Please contribute those to
-[Infiziert90/ChatTwo](https://github.com/Infiziert90/ChatTwo) instead.
+are **not** translated here. They are kept as-is from the last
+upstream sync and remain the work of the Chat 2 Crowdin community.
+Active cherry-picking from upstream ended in the v1.4.x cycle (see
+[`docs/UPSTREAM_SYNC.md`](docs/UPSTREAM_SYNC.md)), so future
+translation improvements to those upstream strings will not flow
+into HellionChat automatically anymore. If you have improvements
+for the original Chat 2 strings, please contribute them to
+[Infiziert90/ChatTwo](https://github.com/Infiziert90/ChatTwo)
+directly.
 
 ## Licensing