feat: rewrite gh-commit in Go with local Ollama backend

Replaces the Python/Crush/DuckDB implementation with a CGO-free Go binary:
cobra CLI, modernc.org/sqlite storage, huh/lipgloss UI, and speed-tuned raw
/api/generate calls (think:false, keep_alive, capped num_predict) against a
local qwen3.5:2b. Ships as a gh extension via cli/gh-extension-precompile.

internal/diff/filter.go

@@ -0,0 +1,101 @@
+// Package diff filters git diffs before they reach the model: lock/generated
+// files lose their hunk bodies, JSON changes are capped, and every other file
+// is truncated. Behavior is a verbatim port of the Python filter_diff.
+package diff
+
+import (
+	"fmt"
+	"regexp"
+	"strings"
+)
+
+var lockPattern = regexp.MustCompile(
+	`(package-lock\.json|yarn\.lock|pnpm-lock\.yaml|bun\.lockb|` +
+		`go\.sum|go\.mod|Cargo\.lock|poetry\.lock|composer\.lock|Gemfile\.lock|` +
+		`.*\.min\.(js|css)|.*\.bundle\.js|.*\.map|` +
+		`dist/.*|build/.*|\.next/.*|node_modules/.*|vendor/.*|__pycache__/.*|\.pyc$|target/.*)`,
+)
+
+var (
+	fileNameRe = regexp.MustCompile(`b/([^ ]+)`)
+	headerRe   = regexp.MustCompile(`^(index|---|\+\+\+|@@)`)
+)
+
+const (
+	// MaxDiffLines caps the per-file line count for regular files.
+	MaxDiffLines = 200
+	// MaxJSONLines caps the +/- line count for JSON files.
+	MaxJSONLines = 50
+)
+
+// Filter rewrites a raw git diff for prompt consumption.
+func Filter(diff string) string {
+	var lines []string
+	inFiltered, inJSON := false, false
+	lineCount, jsonCount := 0, 0
+
+	for _, line := range strings.Split(diff, "\n") {
+		if strings.HasPrefix(line, "diff --git") {
+			lineCount, jsonCount = 0, 0
+			filename := ""
+			if m := fileNameRe.FindStringSubmatch(line); m != nil {
+				filename = m[1]
+			}
+
+			switch {
+			case lockPattern.MatchString(filename):
+				inFiltered, inJSON = true, false
+				lines = append(lines, line)
+				continue
+			case strings.HasSuffix(filename, ".json"):
+				inFiltered, inJSON = false, true
+				lines = append(lines, line)
+				continue
+			default:
+				inFiltered, inJSON = false, false
+			}
+		}
+
+		// Lock/generated files: keep headers, replace each hunk body with a
+		// marker. (The Python original stopped filtering after the first @@,
+		// leaking hunk bodies; the spec requires bodies replaced, so filtering
+		// holds until the next "diff --git".)
+		if inFiltered {
+			if headerRe.MatchString(line) {
+				lines = append(lines, line)
+				if strings.HasPrefix(line, "@@") {
+					lines = append(lines, "[Generated/lock file - content filtered]")
+				}
+			}
+			continue
+		}
+
+		if inJSON {
+			if headerRe.MatchString(line) {
+				lines = append(lines, line)
+				continue
+			}
+			if strings.HasPrefix(line, "+") || strings.HasPrefix(line, "-") {
+				jsonCount++
+				if jsonCount <= MaxJSONLines {
+					lines = append(lines, line)
+				} else if jsonCount == MaxJSONLines+1 {
+					lines = append(lines, fmt.Sprintf("[... JSON truncated after %d lines ...]", MaxJSONLines))
+				}
+			} else {
+				lines = append(lines, line)
+			}
+			continue
+		}
+
+		if lineCount < MaxDiffLines {
+			lines = append(lines, line)
+			lineCount++
+		} else if lineCount == MaxDiffLines {
+			lines = append(lines, fmt.Sprintf("[... truncated after %d lines ...]", MaxDiffLines))
+			lineCount++
+		}
+	}
+
+	return strings.Join(lines, "\n")
+}