Diff length truncation is hardcoded and not user-configurable

[SunYanbox]

Description

Diff length truncation logic is scattered and inconsistent:

Function	Default Max Length	Configurable
getBranchDiff() (line 720)	8000 chars	Hardcoded parameter default
getFilesDiff() (line 788)	8000 chars	Hardcoded parameter default

Two specific problems:

1. 8000 chars is too short for meaningful AI-generated commit messages / PR descriptions on any moderately sized change. Large diffs get truncated heavily, losing context. But without a hard cap, extreme cases (thousands of lines changed) would blow up token usage.

2. No user configuration — users cannot tune the limit based on their project size or AI token budget. The limit should be stored in .quick-pr-studio/settings.json under ProjectSettings.

Affected Code

• src/gitService.ts:720-741 — getBranchDiff() hardcoded maxLength = 8000
• src/gitService.ts:788-845 — getFilesDiff() hardcoded maxLength = 8000
• src/projectConfig.ts — ProjectSettings interface (needs new field)
• src/inputService.ts — callers pass filesDiff to AI without length awareness

Suggested Solution

1. Add maxDiffLength?: number (default 500000) to the ProjectSettings interface in src/projectConfig.ts
2. Load the config value and pass it as maxLength in all getBranchDiff / getFilesDiff calls
3. Also enforce per-file diff limits roughly proportional to the total (e.g., each file gets at most totalLimit / fileCount), so one massive file doesn't starve others
4. Improve the truncation suffix from ...(diff truncated) to include total-available-info (e.g., ...(truncated: showing 500K of 2.1M total diff))
5. Log a warning when truncation occurs so users are aware

No implementation required — design discussion only.