fix(branch): sanitize {{label}} in branch_name_template for git-safe names

[syf2211]

Summary

Sanitize GitHub label values before substituting them into branch_name_template, so scoped labels like area:permissions no longer produce branch names containing : that fail validateBranchName and abort the action run.

Motivation

Fixes #1491. Labels are free-form and often scoped with : or / (e.g. area:permissions, provider:bedrock). The {{description}} variable was already normalized to kebab-case, but {{label}} was substituted verbatim. A template like {{prefix}}{{label}}/{{entityNumber}} could crash the entire run with process.exit(1).

Changes

• src/utils/branch-template.ts: add shared sanitizeBranchSegment() helper; route extractDescription() and new sanitizeLabel() through it
• generateBranchName(): substitute sanitized label, falling back to entityType when sanitization yields empty
• test/branch-template.test.ts: regression tests for scoped labels, colon-containing labels, and empty-sanitize fallback; assert validateBranchName passes

Tests

• bun test test/branch-template.test.ts — pass (23/23)
• bun test — pass (772/772)
• bun run typecheck — pass
• composer-2.5 review: COMMENT (addressed JSDoc + fallback test)

Notes

• Intentionally scoped to {{label}} only; custom templates with literal punctuation in {{prefix}} are unchanged (pre-existing behavior).
• Labels that sanitize to empty (e.g. :::) fall back to entityType, matching the documented fallback when no label is provided.

Fixes #1491

[ashwin-ant]

Thanks. #1492 fixes the same {{label}} sanitization issue and merged first, so closing this as a duplicate.