Git SCM: Hidden commit input should still support editor-first template authoring before commit

# 1. Workflow intent:
- I want git.showCommitInput=false to reclaim SCM space.
- I still want clicking SCM Commit to open a commit editor with my template before commit finalises.

# 2. Current behaviour:
- With hidden commit input and commit template configured, Commit can finalise immediately using template text.
- No editor prompt appears, so context-rich message authoring is bypassed.

# 3. Expected behaviour:
- New mode/setting: when commit input is hidden, SCM Commit should always open commit editor first (never finalise directly from template text).
- Optionally support template insertion without immediate commit finalisation.

# 4. Why this matters:
- UI-first users and AI-assisted commit authoring need a deterministic review/edit step.
- Current behaviour encourages low-quality “template-only” commits.

# 5. Environment:
VS Code version:
```
Version: 1.132.0 (user setup)
Commit: df53daabb18cd157bdb08c7f01c34df936cf12f4
Date: 2026-08-04T15:30:20Z
Electron: 42.7.1
ElectronBuildId: 14760073
Chromium: 148.0.7778.280
Node.js: 24.18.0
V8: 14.8.178.38-electron.0
OS: Windows_NT x64 10.0.19045
```
Git version: git version 2.51.2.windows.1
OS (Windows): 10.0.19045 Build 19045