Cleanup pass 2: Fix .gitignore corruption, archive docs/audit + phase10 helpers, fix PROJECT_DESCRIPTION.md

ui: Clean up editor placeholder text and alignment (top-right)

docs: NLP-3.5 Hardening Sprint — Complete audit reports (6 docs) - hardening-final-report.md: Full sprint summary (9/9 tasks done) - analyze-stress-test.md: 50-2000 chars, all pass - nlp-performance-breakdown.md: AraSpell ~700ms/word bottleneck - suggestion-priority-audit.md: grammar(3)>punc(2)>spell(1)>auto(0) - overlap-resolution-report.md: 4/4 overlap tests clean - text-processing-strategy.md: Skip AraSpell for >300 chars 500 chars: TIMEOUT→28s | Overlaps: 1→0 | API: backward compatible

docs: Phase O Final Hardening — 6 audit reports - phase-o-final-hardening.md: Known limitations investigated - api-contract-audit.md: Full API schema documentation - model-performance-report.md: RAM, latency, throughput benchmarks - frontend-robustness-report.md: 42/42 UI elements verified - security-audit.md: RLS, JWT, CORS, input validation - architecture-consistency-audit.md: 95% code-docs consistency Key findings: 500-char timeout (AraSpell bottleneck), grammar+punctuation dedup gap