fix: preserve original indentation character in /improve suggestions

[Oxygen56]

What

Fixes #1858 — /improve replaces tabs with spaces in tab-indented codebases (Go, Makefile, etc.)

Root Cause

The dedent_code method in pr_code_suggestions.py only adjusted indentation when delta_spaces > 0 and never normalized the suggestion indentation character to match the original file.

Fix

1. Handle delta_spaces < 0 (suggested code has more indent than original) with textwrap.dedent
2. When the original file uses tabs, normalize all suggestion lines to use tabs (4 spaces → 1 tab)
3. Auto-detection from original file content — no configuration needed

Before

func foo() {
    // suggested code uses spaces ← breaks Go formatting  
    return bar()
}

After

func foo() {
\t// suggested code uses tabs ← preserves original formatting
\treturn bar()
}