docs(agents): add host contribution guide

[phodal]

Summary

Add one evidence-first contribution workflow for supporting a new Coding Agent
host. The guide separates native discovery, configured assets, session evidence,
shared registration, output routing, and packaging so contributors can land
partial support without overstating coverage.

The guide links PR #6 (Qwen Code) and PR #22 (GitHub Copilot) as contrasting
worked examples while making current repository contracts the source of truth.
It is discoverable from AGENTS.md, CONTRIBUTING.md, the English and Chinese
READMEs, Community Extensibility, the canonical Host Adapter Matrix, and new
English/zh-Hans Docusaurus pages.

Why

• Issue/Story: None; this is justified documentation maintenance requested to
  turn two host contributions into a reusable contributor route.
• User or maintainer outcome: Contributors can identify which host-support
  slices they are claiming, find their canonical owners, and attach appropriate
  native, privacy, cross-platform, test, packaging, and review evidence.

Traceability and Scope

• Spec: docs/specs/2026-07-30-new-coding-agent-contribution-guide.md
• Acceptance criteria: NCAG-AC-1 through NCAG-AC-6
• Canonical owners changed: docs/adapters/, contributor entry pages, and the
  curated English/zh-Hans Hosts site routes.
• Explicit non-goals: no host adapter, provider enum, manifest, session parser,
  report mode, packaging target, PR feat(host): add Qwen Code host adapter #6 change, or PR feat(copilot): add GitHub Copilot as a first-class host platform #22 change.

Change Type

• Feature
• Bug fix
• Tests only
• Documentation/community
• Refactor with no intended behavior change
• Dependency, packaging, or infrastructure

Test and Review Evidence

Check	Result
node scripts/doc-link-graph/cli.mjs skills/better-harness	34 files / 50 links; no tracked graph drift
node --test test/doc-link-graph.test.mjs test/docs-site.test.mjs	7/7 passed
npm ci && npm run build from docs/	English and zh-Hans built with zero broken links
npm test	867/867 passed
npm run pack:verify	Passed; npm 311 entries, runtime ZIP 337 entries
git diff --check	Passed

Manual evidence: inspected every new navigation route and both external PR
links. No visual grammar or layout changed, so no new screenshot is included.

Risk and Recovery

• Compatibility and cross-platform impact: Documentation only. The guide
  explicitly requires Windows, macOS, and Linux evidence proportional to future
  host changes.
• Package, plugin, schema, or generated-file impact: no plugin, schema, runtime,
  dependency, or generated graph changes. Canonical repository Markdown may be
  present in documented package roots; Docusaurus source remains excluded by
  existing package boundaries.
• Rollback or recovery path: revert commit 4b6f722.
• Residual risk: host contracts and example PRs can evolve. The guide requires
  contributors to verify current host versions and defers to current repository
  owners. npm ci reported audit findings from the unchanged docs lockfile; this
  PR does not change dependencies.

AI Involvement

• Level: Generated
• Human review and validation: Not yet performed; this Draft PR records the
  automated and source-backed checks completed by Codex for maintainer review.

Checklist

• I followed AGENTS.md, CONTRIBUTING.md, and the relevant canonical-owner guidance.
• The change is focused and does not include unrelated local or generated state.
• Tests and documentation match the behavior actually delivered.
• Markdown links were checked when documentation moved or changed.
• Cross-platform behavior was considered for Windows, macOS, and Linux.
• Package/runtime verification was run when shipped files or dependencies changed.
• CHANGELOG.md is not required because this adds contributor guidance without changing user-facing runtime behavior.
• The contribution is submitted under the repository's MIT License.