docs(agents): consolidate durable engineering notes into scoped AGENTS.md

[CybotTM]

Consolidates durable, verified engineering notes into the four scoped AGENTS.md files. Docs-only, no code changes.

Each fact was checked against the codebase before writing; several stale or incorrect drafts were dropped or corrected — e.g. the API is /api/v2/* with a {error, message} shape (not Nelmio / /api/v1 / RFC 9457), the ExtJS→SolidJS migration is already complete (so no "ongoing" note), and the merge gate does include the required CI Success aggregate check.

• frontend/AGENTS.md — Ark UI date-picker / inline-editor focus pattern, "Philosophy A" grid-editing architecture, stack decisions (Ark UI vs Kobalte, Tailwind 4, no Vue/Svelte), vitest-axe/jsdom note.
• e2e/AGENTS.md — db-e2e reseed procedure, image bind-mount rebuild flow, TestClock vs APP_FROZEN_TIME, usable test users, save-round-trip timeout guidance.
• tests/AGENTS.md — portable LIKE … ESCAPE, logInSession() EM identity-map validator gotcha.
• src/AGENTS.md — SaveDto grandfathering pattern, Holiday DBAL-only entity, self-migrating prod entrypoint, and a new "Deployment, CI & merge (operational)" section.

[gemini-code-assist]

Code Review

This pull request updates the documentation in several AGENTS.md files across the repository (e2e, frontend, src, and tests). The updates provide detailed instructions and architectural guidelines on database seeding, rebuilding after code changes, test environment configurations, frontend stack decisions (such as Ark UI and SolidJS), custom date picker implementations, backend validation rules, DBAL usage for specific entities, CI/CD deployment workflows, and testing best practices (including SQLite compatibility and functional test limitations). There are no review comments, so I have no feedback to provide.

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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