feat(ctrl): improve template errors and GitHub URL handling

[akashgreninja]

closes #

📑 Description

✅ Checks

• I have tested my code (provide screenshots or screen recordings of a working solution)
• I have performed a self-review of my code
  No breaking changes; behavior unchanged apart from error text.

Feature 1
1)Normalize GitHub repo URLs by stripping .git.

Screen.Recording.2025-10-03.at.10.38.07.PM.mov

2. Clear, actionable errors for invalid path (must be chart root). Add User-Agent and safe Accept for GitHub tarball fetch.

3. Bad commit/tag → clear ref-not-found message.

4. Private repo checks

ℹ Additional context

[akashgreninja]

@petar-cvit please can i get a review ?