Add arch plugin (architecture + modernization skills) (#2253)

* Add arch plugin (architecture + modernization skills)

Add the `arch` plugin with two skills:
- `arch:document` — produce a single, cited architecture document for a
  locally-cloned repo, reading files on disk only.
- `arch:modernize` — generate a phased modernization plan, auto-running the
  document workflow first when no architecture doc exists.

Skill sources live in top-level skills/ and are referenced declaratively
from plugins/arch/.github/plugin/plugin.json, per the repo's plugin model.
Regenerated docs/README.plugins.md, docs/README.skills.md and marketplace.json.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Merge arch skills into single doc-and-modernize skill

Collapse the document and modernize skills into one standalone skill
(doc-and-modernize) with Documentation and Modernization modes, keeping
the plugin named arch. Modernization mode now runs the Documentation
workflow inline instead of invoking a separate arch:document skill,
fixing standalone-install cross-skill references. Reframe Documentation
mode as local-first (remote/API lookups are a flagged last resort)
rather than local-only. Regenerate docs and marketplace.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address PR review: list skill by bare ID in arch README

Use 'doc-and-modernize' (repo convention) instead of the namespaced
'arch:doc-and-modernize', noting it surfaces as arch:doc-and-modernize
when installed via the plugin.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address PR review: use non-HTML placeholder in instructions template

Change plain-text <PROJECT NAME> to [PROJECT NAME] in the header and
first paragraph so Markdown renderers don't parse it as an HTML tag and
drop it. The code-span `<N>` on the phase line is unaffected.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address PR review + fix codespell CI failure

- Fix codespell: pre-empts -> preempts in the instructions template
- Consistent terminology: replace 'research step/workflow' with
  'Documentation mode' in SKILL.md, README, and plugin.json description
- Fix run-on: add 'that' before 'Modernization mode must surface'
- Regenerate docs/marketplace for the updated plugin description

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* Address PR review feedback

- Instruct redacting credentials/tokens from git remote URLs before recording
- CI enforcement: ask user or mark [UNVERIFIED]; remote lookup is flagged last resort

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: samqbush <samqbush@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

plugins/arch/.github/plugin/plugin.json

@@ -0,0 +1,20 @@
+{
+  "name": "arch",
+  "description": "Architecture and modernization toolkit: produce a cited architecture document for a locally-cloned repo, and generate a phased modernization plan that auto-runs Documentation mode when needed.",
+  "version": "1.0.0",
+  "keywords": [
+    "architecture",
+    "modernization",
+    "documentation",
+    "migration",
+    "onboarding"
+  ],
+  "author": {
+    "name": "Awesome Copilot Community"
+  },
+  "repository": "https://github.com/github/awesome-copilot",
+  "license": "MIT",
+  "skills": [
+    "./skills/doc-and-modernize/"
+  ]
+}