docs: reorganize tree, restore architecture + research from archive, add Phase 0 reports

Active docs/ was stub scaffolding while the real docs sat in docs/_archive/.
Restore and correct them, and record the Phase 0 work.

- docs/architecture/current.md: verified system map (from archived SYSTEM_OVERVIEW,
  9 corrections against code).
- docs/research/competitors.md: sourced competitor analysis (from archived
  PRODUCT_RESEARCH, feature matrix corrected).
- docs/decisions/ADR-002-job-application-model.md: the Job/JobApplication split.
- docs/application-discovery-report.md, docs/implementation-roadmap.md,
  docs/phase-0-foundation-report.md, docs/career-workspace-branch-assessment.md.
- Remove 10 zero-byte placeholder files that advertised content that never existed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/ai/providers.md

@@ -0,0 +1,66 @@
+# AI Providers
+
+## Purpose
+
+Support multiple AI providers.
+
+---
+
+# Current/Future Providers
+
+Possible:
+
+- OpenAI.
+- Anthropic.
+- Google Gemini.
+- Groq.
+- Ollama.
+- Other compatible APIs.
+
+---
+
+# Provider Architecture
+
+Use abstraction:
+
+AI Provider Interface
+
+↓
+
+Provider Implementation
+
+↓
+
+Model
+
+---
+
+# Requirements
+
+Users should eventually be able to:
+
+- Select provider.
+- Select model.
+- Configure API keys.
+
+---
+
+# Admin Controls
+
+Admins should control:
+
+- Available providers.
+- Allowed models.
+- Usage limits.
+
+---
+
+# SaaS Consideration
+
+Different plans may have:
+
+Free models.
+
+Premium models.
+
+BYO-key support.