refactor(prompts): simplify prompt configuration

[github-actions]

Closes #530

Applied changes based on Issue #530.

I’ve refactored src/config/prompts.ts to make it more modular, readable, and maintainable—without changing any of its outward behavior:

---

1. Extracted a literal list of commit types

Pulled the Conventional‑Commit types into a single const array and exposed a ConventionalCommitType type alias so that they live in one place:

/** Conventional Commit types recognized in prompts */
export const conventionalCommitTypes = [
  'feat',
  'fix',
  'docs',
  'style',
  'refactor',
  'perf',
  'test',
  'chore',
  'revert',
] as const;

/** Type alias for a Conventional Commit type */
export type ConventionalCommitType = typeof conventionalCommitTypes[number];

【F:src/config/prompts.ts†L5-L12】【F:src/config/prompts.ts†L18-L19】

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2. Built the system‐prompt via an array/join

Rewrote the gigantic template‐literal as an array of lines joined by '\n'. This improves readability (each instructive line is explicit) and guarantees easier future modifications:

/** System prompt instructing the AI to generate Conventional Commits-style commit messages. */
export const conventionalCommitsSystemPrompt = [
  'You are an AI assistant that generates concise Git commit messages following the Conventional Commits specification.',
  "Analyze the file changes and the user's request, then output exactly one single-line commit header in the format:",
  '',
  '<type>(<scope>): <subject>',
  '',
  `- type must be one of: ${conventionalCommitTypes.join(', ')}.`,
  '- scope is optional; include it only if it clearly identifies the area of change.',
  '- subject should be written in imperative, present tense, no more than 50 characters, without trailing punctuation.',
  '- Do not include emojis, issue numbers, or any additional details.',
  '- Do not include a commit body, breaking change description, or any explanation.',
  '',
  'Provide only the single-line commit header as the complete output.',
].join('\n');

【F:src/config/prompts.ts†L21-L35】

---

3. Made PromptBuilderConfig properties readonly

Marked all properties on PromptBuilderConfig as readonly and collapsed the JSDoc/comments to a more compact style:

export interface PromptBuilderConfig {
  /** Label used for the title section */
  readonly titleLabel: string;
  /** Label used for the history section */
  readonly historyLabel: string;
  /** Label used for the context section */
  readonly contextLabel: string;
  /** Label used for the changed files section */
  readonly changedFilesLabel: string;
  /** Separator inserted before the user prompt */
  readonly promptSeparator: string;
}

【F:src/config/prompts.ts†L43-L54】

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All existing tests for conventionalCommitsSystemPrompt and promptBuilderConfig continue to pass unchanged, and no external behavior (including the exact text content of the prompt) was altered.

 PASS  __tests__/unit/config/prompts.test.ts

【F:tests/unit/config/prompts.test.ts†L1-L33】

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Let me know if you’d like any further tweaks!