Add arch plugin (architecture + modernization skills)

[samqbush]

What this adds

A new curated in-repo plugin, arch, providing a single consolidated skill for working with locally-cloned repositories:

• doc-and-modernize (surfaces as arch:doc-and-modernize when installed via the plugin) — one skill with two complementary modes:
  • Documentation mode — Produce a single, comprehensive, cited architecture document, local-first: it reads the local checkout by default, cites every claim to a file+line, flags unverified facts, and deep-dives the most complex subsystems. A web/API lookup is a deliberate, clearly-flagged last-resort fallback for a fact that genuinely can't be determined from disk (marked [UNVERIFIED]/sourced-remotely), not the default path. Great for onboarding docs.
  • Modernization mode — Generate a phased modernization plan for a legacy codebase. If an architecture document already exists it builds on it; otherwise it first runs the Documentation-mode workflow inline, then continues to the plan. Produces per-feature migration docs, tech-stack ADRs, and an adaptive, safety-laddered phased plan.

Structure

Following the repo's declarative plugin model, the skill source lives in the top-level skills/ directory and is referenced from plugins/arch/.github/plugin/plugin.json; CI materializes it into the installable plugin. The two workflows were consolidated into one standalone skill (rather than two) because top-level skills/ are published/validated as standalone, so the modes now cross-reference each other inline instead of invoking a separate skill.

• plugins/arch/.github/plugin/plugin.json + plugins/arch/README.md
• skills/doc-and-modernize/SKILL.md (+ references/)
• Regenerated docs/README.plugins.md, docs/README.skills.md, .github/plugin/marketplace.json

Validation

• npm run skill:validate ✅
• npm run plugin:validate ✅
• npm run build (regenerated docs + marketplace) ✅
• bash eng/fix-line-endings.sh ✅

[github-actions]

🔒 PR Risk Scan Results

Scanned 8 changed file(s).

Severity	Count
🔴 High	0
🟠 Medium	1
ℹ️ Info	0

Severity	Rule	File	Line	Match
🟠	package-exec-command	docs/README.skills.md	31	| [acreadiness-assess](../skills/acreadiness-assess/SKILL.md)<br />`gh skills install github/awesome-copilot acreadiness-assess` | Run the AgentRC readiness assessment on the curre

This is an automated soft-gate report. Findings indicate review targets and do not block merge by themselves.

[github-actions]

🔍 Vally Lint Results

⛔ Findings need attention

Scope	Checked
Skills	1
Agents	0
Total	1

Severity	Count
❌ Errors	1
⚠️ Warnings	0
ℹ️ Advisories	0

Summary

Level	Finding
❌	doc-and-modernize (1/2 checks passed, 1 failed)

Full linter output

### Linting skills/doc-and-modernize
npm warn EBADENGINE Unsupported engine {
npm warn EBADENGINE   package: 'commander@15.0.0',
npm warn EBADENGINE   required: { node: '>=22.12.0' },
npm warn EBADENGINE   current: { node: 'v20.20.2', npm: '10.8.2' }
npm warn EBADENGINE }
npm warn deprecated prebuild-install@7.1.3: No longer maintained. Please contact the author of the relevant native addon; alternatives are available.
❌ doc-and-modernize (1/2 checks passed, 1 failed)
    ✗ [spec-compliance] 1 of 1 skill(s) have spec violations.
        ✗ spec-compliance: Spec checks failed.
            ✗ File length (851 lines) exceeds limit (500).
    ✓ [valid-refs] All file references across 1 skill(s) are valid.
        ✓ valid-refs: All file references resolve to existing files within the skill directory.

1 skill(s) linted, 1 failed

Note: Vally lint returned a non-zero exit code. Please review the findings above before merge.

[Copilot]

Pull request overview

Adds a new in-repo plugin, arch, that bundles two new skills to (1) generate a cited architecture document from a local checkout and (2) produce an adaptive, phase-based modernization plan (including supporting reference materials). Updates the generated plugin/skill docs and marketplace index so the new resources are discoverable and installable.

Changes:

• Added two new skills: document (architecture research) and modernize (modernization planning with safety-ladder framing).
• Added plugins/arch with plugin manifest + README bundling the two skills.
• Updated generated documentation (docs/README.*) and .github/plugin/marketplace.json to include the new plugin/skills.

Show a summary per file

File	Description
skills/modernize/SKILL.md	Defines the modernization planning workflow, phases, and governance model.
skills/modernize/references/migration-hazards.md	Adds a reusable hazard catalog (H1–H8) to red-team migration phases.
skills/modernize/references/copilot-instructions.template.md	Adds a template for .github/copilot-instructions.md to execute the plan.
skills/document/SKILL.md	Defines the local-checkout architecture research workflow and output structure.
plugins/arch/README.md	Documents the new arch plugin and its included skills.
plugins/arch/.github/plugin/plugin.json	Registers the new arch plugin and bundles the two skills.
docs/README.skills.md	Adds entries for the new document and modernize skills.
docs/README.plugins.md	Adds the new arch plugin to the plugin index.
.github/plugin/marketplace.json	Adds arch to the generated marketplace listing.

Review details

• Files reviewed: 9/9 changed files
• Comments generated: 4
• Review effort level: Low

[Copilot]

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 3
• Review effort level: Low

[Copilot]

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 1
• Review effort level: Low

[Copilot]

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 4
• Review effort level: Low

[Copilot]

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 2
• Review effort level: Low

[Copilot]

Review details

• Files reviewed: 8/8 changed files
• Comments generated: 0 new
• Review effort level: Low