docs(agents): capture review-derived coding patterns

[AndyScherzinger]

Extends AGENTS.md with coding patterns distilled from the review feedback by @enjeck and @samin-z on the personal-archiving PR #2476, so future contributions get these details right from the start instead of rediscovering them in review.

Added to the Architecture Patterns section:

• RequirePermission for contexts — the middleware only accepts integer node types for tables/views; context endpoints must use typeParam: 'context', never an integer NODE_TYPE_CONTEXT.
• node_type column types differ per table — tables_shares.node_type stores strings while other tables store integer constants; always convert via ConversionHelper, never compare directly.
• Per-user resolved state in API responses — every endpoint returning an entity with per-user overrides (including single-item GETs) must resolve the value for the requesting user.
• Migration file naming — rename a long-lived PR's migration to the current target app version and a current date before merge.
• Cleanup of materialized per-user state on access loss — which listeners to wire (group/circle member removal, group/circle deletion, share deletion, user deletion) and to verify remaining access via PermissionsService first.
• Atomic upserts — use the portable IDBConnection::setValues() instead of read-modify-write or vendor-specific SQL.

Added to the Frontend section:

• Guard findIndex() results against -1 in Pinia store actions; on Vue 3, direct array index assignment is reactive, so no splice() workarounds.

🖼️ Screenshots

Not applicable — documentation-only change (AGENTS.md).

🏁 Checklist

• ⛑️ Tests (unit and/or integration) are included or not needed
• 🔙 Backport requests are created or not needed: /backport to stableX.X
• 📅 Milestone is set
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🤖 AI (if applicable)

• The content of this PR was partly or fully generated using AI

AI disclosure: This change was AI-assisted — written with Claude Code (model claude-fable-5) and reviewed by the human contributor before submission.

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