[DOCS] Add Agents.md, skills and contexts for ai coding assistant

[VincentG1234]

Summary

Add structured documentation for human maintainers and coding agents: AGENTS.md, .ai/context/, .ai/skills/, and a user-facing architecture guide with Mermaid diagrams. Link the new doc from README and quick start. Align the local example YAML with vLLM V1 constraints.

Why

The InseeFrLab fork needs a stable onboarding path for contributors and agents (local backend focus, safe commands, known issues/PRs) without reading the whole codebase. Architecture was previously only implicit in code; diagrams improve onboarding and reviews.

What changed

• AGENTS.md — entry point for agents (context files, skills, safe commands).
• .ai/context/ — repo map, execution flow, history, known issues, current work snapshot, external links.
• .ai/skills/ — pr-writer, pr-reviewer, test-writer, docs-writer, architecture-diagrams.
• docs/architecture.md — end-to-end, layout, orchestration, trial lifecycle, outputs (Mermaid).
• README.md, docs/quick_start.md — links to architecture doc.
• examples/study_config_local_exec.yaml — disable max_num_partial_prefills (unsupported on V1; comment added).

How tested

• ruff check .
• pytest -v tests/
• Manual E2E (maintainer): not required (docs-only PR)

Risks / limitations

• .ai/context/current-work.md is a point-in-time snapshot (open PRs [FIX] local backend trial cleanup #13, [FIX] add guidellm preflight check and pin vllm to <=0.19 #17, [FIX] Do not charge baseline trials against the optimization n_trials budget (+ test) #21, [FEAT] add log_metrics as Optuna trial user attrs #22); it will drift until refreshed after merges.
• Mermaid rendering depends on the viewer (GitHub, IDE); no runtime behavior change except the example YAML default.

Links

• (none — no issue closed)