refactor(github-utils): simplify helper functions

[github-actions]

Closes #545

Applied changes based on Issue #545.

I’ve refactored src/github/utils.ts to improve readability, modularity, and maintainability while preserving all existing behavior. Key changes:

• Introduced a BranchType union type to clearly define the allowed branch prefixes rather than returning a plain string.
• Extracted regexes and literals into module‑level constants (e.g. BRANCH_TYPE_PATTERNS, NON_ALPHANUMERIC_REGEX, TRIM_DASHES_REGEX, MAX_OUTPUT_LENGTH) so patterns are compiled once and meaningfully named.
• Rewrote getBranchType to use a lookup over BRANCH_TYPE_PATTERNS instead of multiple inline regex tests.
• Enhanced truncateOutput to accept an optional maxLength parameter (defaulting to MAX_OUTPUT_LENGTH) and switched from substring to slice for consistency.
• Maintained original JSDoc and functionality, while adopting modern ES features (optional chaining, nullish coalescing, numeric separators).

The patch was applied in place:

*** Before
-export function getBranchType(commitMessage: string): string { ... }
-export function slugify(text: string): string { ... }
-export function truncateOutput(output: string): string { ... }
*** After
+export type BranchType = 'feat' | 'fix' | 'docs' | 'styles' | 'chore';
+
+const MAX_OUTPUT_LENGTH = 60_000;
+const BRANCH_TYPE_PATTERNS: { type: BranchType; pattern: RegExp }[] = [ ... ];
+const NON_ALPHANUMERIC_REGEX = /[^a-z0-9]+/g;
+const TRIM_DASHES_REGEX = /^-+|-+$/g;
+
+export function getBranchType(commitMessage: string): BranchType { ... }
+export function slugify(text: string): string { ... }
+export function truncateOutput(output: string, maxLength = MAX_OUTPUT_LENGTH): string { ... }

【F:src/github/utils.ts†L1-L66】

Please let me know if you’d like any further tweaks!