docs: analyze prompt storage strategy for prompt window

[Idddd]

Motivation

• Provide a concise baseline analysis of how prompts are persisted in the project to inform design and implementation of a dedicated prompt window feature.

Description

• Add doc/prompt_storage_strategy_analysis.md which documents current prompt persistence (YAML files .ai-test.yml/.ai-test.yaml), built-in defaults in AiPromptDefaults, profile format and resolution (PromptProfileSet and PromptProfileResolver), write-back behavior via LlmSettingsLoader.buildPromptsMap, runtime selection persistence in prompt menu actions, risks, and recommendations; no production code changes were made and a new branch branch/prompt_window was created for this work.

Testing

• No automated unit tests were changed or required for this documentation-only update, and repository operations used to validate the change succeeded (e.g. git checkout -b branch/prompt_window, git add + git commit, and git status).

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Codex Task