docs: add README hero and social preview

[YSCJRH]

Summary

• Replace the README Mermaid first-screen diagram with committed raster hero artwork.
• Add a matching GitHub social preview asset and prompt contract for future regeneration/review.
• Update the project presentation checklist with asset paths and visual safety constraints.
• Bump patch metadata to 0.2.25 for the Phase 1 productization release.

Validation

• python -m pip install -e ".[dev]"
• python -m pytest -q -> 316 passed, 1 skipped
• python harness/scripts/run_mcp_smoke.py -> PASS
• python -m winchronicle codex install --dry-run -> PASS
• GITHUB_HEAD_REF=codex/p1-readme-hero-social python harness/scripts/check_productization_scope.py -> PASS for 11 changed files
• git diff --check origin/main...HEAD -> PASS
• Asset size check: winchronicle-hero.png is 1400x720; winchronicle-social-preview.png is 1280x640

Isolated Review Reports

• Privacy Boundary Reviewer: PASS. No screenshots, OCR, keyboard logging, clipboard capture, audio, cloud upload, desktop control, daemon/service, infinite polling, MCP write tools, arbitrary file reads, network upload, product-targeted capture, official-brand implication, or real observed content found.
• Product Clarity Reviewer: PASS after assets were committed. README now has product promise, hero image, caption, and Chronicle comparison no longer dominates the first screen.
• Release Gate Reviewer: PASS after assets were committed. Scope is limited to P1 docs/assets plus release metadata; version metadata is consistent at 0.2.25.

Privacy Boundary

This PR changes presentation assets and release metadata only. It does not add or imply default screenshots, OCR, clipboard capture, keylogging, audio recording, cloud upload, desktop control, background daemon/service, infinite polling, MCP write tools, arbitrary file reads, network upload, product-targeted capture behavior, or official OpenAI affiliation. Observed UI content remains untrusted observed data.