chore(hygiene): remove legacy SecondBrain residues, generate social preview, and link Project v2 in README

[nicolasvd]

Closes #22

📌 Summary

Branch: chore/issue-22-remove-legacy-secondbrain-residues-gener → main
Type: chore(hygiene)
Related Issue: #22 chore(hygiene): remove legacy SecondBrain residues, generate social preview, and link Project v2 in README

• Residual SecondBrain Cleanup: Completely removed legacy application references across theming playbooks, test contracts, and scripts, normalizing naming to AgenticAndroidStarter and KernelApp.
• Social Preview Generation: Created official 1280x640 vector asset (social-preview.svg) and rendered pixel-perfect PNG (social-preview.png) following the Serene Intellectual design system with a 40pt safe border.
• Live Delivery Board Integration: Added a dedicated section and status badge in README.md linking directly to the live GitHub Project v2 Kanban board (Agentic Delivery Board #2).

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📂 Affected Files

File	Change	Role
social-preview.svg	+180 -0	Official GitHub Social Preview vector asset (1280x640)
social-preview.png	+Binary	High-fidelity rendered PNG preview for repository showcase
README.md	+12 -1	Added Live Kanban Board badge and dedicated section
.agent/playbooks/compose-theming.md	+2 -2	Normalized theme entry point to AgenticAndroidStarterTheme
.agent/playbooks/roborazzi-export.md	+2 -2	Normalized screenshot test theme wrapper
.github/ISSUE_TEMPLATE/crashlytics_triage.yml	+1 -1	Normalized crash stacktrace placeholder package
scripts/upload-screenshots.py	+1 -1	Normalized test class in help instruction
design-system.md	+2 -2	Normalized TopBar and KernelApp component names

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✅ Quality Airbag — ./scripts/quality-check.sh

Check	Result
Kotlin Progressive compilation	✅ 0 errors
Android Lint (debug)	✅ 0 errors / 0 warnings
Android Lint (release)	✅ 0 errors / 0 warnings
Unit & Roborazzi screenshot tests	✅ 100% passing
Documentation contracts (validate-docs.sh)	✅ 38/38 valid (11 categories OK)
Runtime Guardrails (test-runtime-guardrails.mjs)	✅ 100% passed (PoLP, Plan Guard, Branch Guard)

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🔒 Gate 3.5 — Auto-Merge Lock

Important

Zero Auto-Merge Guardrail: This pull request will NOT be merged autonomously. It halts here awaiting explicit written confirmation from the user ("Tu peux merger").