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/security/ai-security.md

@@ -0,0 +1,35 @@
+# AI Security
+
+## Purpose
+
+Protect against AI-related risks.
+
+---
+
+# Prompt Injection
+
+Untrusted content:
+
+- Job descriptions.
+- Uploaded CVs.
+- Emails.
+
+must not control AI behaviour.
+
+---
+
+# AI Output Validation
+
+Check:
+
+- Unsupported claims.
+- Dangerous content.
+- Formatting issues.
+
+---
+
+# Data Handling
+
+Only send required information.
+
+Avoid unnecessary exposure.