feat(plugin): complete SimHub POC — PKCE auth, minimal UI, E2E docs

[wgutmann]

• API: GET /api/auth/config for Discord client ID (PKCE)
• Plugin: PkceHelper, browser PKCE flow, WinPodiumsSettingsControl (WPF)
• PluginMain: IWPFSettingsV2, AuthenticateWithBrowserAsync
• Docs: manual E2E steps in development.md, next-steps link

Summary

• [What changed]
• [Why it changed]

Traceability (ContextStream / knowledge graph)

Required. Link this PR to the tech plan(s) and PRD so the ContextStream knowledge graph can show PR ↔ Tech Plan ↔ PRD. Use the stable doc IDs from .github/traceability-mapping.yaml.

• Implements (Tech Plan): e.g. TP-SPOC-001 — [list one or more TP-XXX IDs this PR implements, or "none" if doc-only / infra]
• PRD: e.g. PRD-001 — [PRD ID this work traces to, or "none"]
• Doc links: Add markdown links to the PRD and tech plan doc(s) using the traceability mapping. Example: PRD-001, TP-SPOC-001. (Use "none" if doc-only / infra.)
• Labels: Apply traceability labels from .github/labels.yaml (e.g. prd:PRD-001, tech-plan:TP-SPOC-001).

Risk

• [low / medium / high] — [brief rationale]

Test plan

• [test or verification step]

Rollback

• [How to revert if needed]

Product impact

• Does this change scope, requirements, or user-facing copy? (e.g. docs/product/**, docs/brand/**, UI strings, API descriptions, landing copy)
  • If yes: Product review requested. Use the PM review checklist (human PM or agent with product manager personality).
  • If no: Leave unchecked and skip.

[github-actions]

PR traceability check (recommended for ContextStream PR ↔ Tech Plan ↔ PRD):

• PR must have at least one traceability label from .github/labels.yaml (e.g. prd:PRD-001, tech-plan:TP-SPOC-001).

See CONTRIBUTING and the PR template.