docs: restructure into docs/ folder, add roadmap and learning notes

- Move AI_DISCLOSURE, THIRD_PARTY_NOTICES, UPSTREAM_SYNC, ipc.md
  into docs/ (ipc.md renamed to IPC.md for consistency)
- Add docs/ROADMAP.md, docs/CHANGELOG.md, docs/CONTRIBUTORS.md,
  docs/LEARNING-JOURNEY.md
- Update README to reflect the v1.0.0 standalone state, drop the
  development section, refresh the architecture tree, add a
  release-cadence block linking to LEARNING-JOURNEY
- Fix stale ChatTwo/* source paths to HellionChat/* across docs
- Update cross-links in PRIVACY, CONTRIBUTING and .github/* so they
  point at the new docs/ paths

Pure documentation pass, no code changes.

.github/PULL_REQUEST_TEMPLATE.md

@@ -53,7 +53,7 @@ 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 UPSTREAM_SYNC.md.
+- Does this affect the upstream cherry-pick path? See docs/UPSTREAM_SYNC.md.
 -->
 
 ## Checklist
@@ -67,6 +67,6 @@ new commands, new translations, removed behaviour. If none, write
 - [ ] I updated the README, in-plugin strings or documentation if my
       change is user-visible.
 - [ ] I did not include any AI-generated code without disclosing it
-      in the PR description (see [AI_DISCLOSURE.md](../AI_DISCLOSURE.md)).
+      in the PR description (see [AI_DISCLOSURE.md](../docs/AI_DISCLOSURE.md)).
 - [ ] I confirm my contribution is released under the
       [EUPL-1.2](../LICENSE).