fix: make bd onboard output generic, not beads-specific (#465)

The copilotInstructionsContent in onboard.go contained content specific to
developing the beads project itself (Go tech stack, SQLite, Cobra, project
structure, release scripts, etc.). This content doesn't make sense when
running bd onboard in other repositories that use bd for issue tracking.

Removed:
- Tech Stack section (Go, SQLite, Cobra, etc.)
- Coding Guidelines section (beads development instructions)
- Project Structure section (beads directory tree)
- Scripts section (bump-version.sh, release.sh, etc.)
- Key Documentation section (beads-specific docs)
- Beads-specific rules (BEADS_DB testing, test issues in production DB)

Kept:
- All bd command examples and workflow (unchanged)
- Priorities and issue types
- MCP server instructions
- Generic rules applicable to any project using bd

The beads repo's own .github/copilot-instructions.md is manually maintained
and still contains the beads-specific development instructions.

cmd/bd/onboard_test.go

@@ -153,9 +153,9 @@ func TestGenerateBDGuide(t *testing.T) {
 		// Verify key sections from copilotInstructionsContent are present
 		expectedSections := []string{
 			"GitHub Copilot Instructions",
-			"Project Structure",
-			"Tech Stack",
-			"Coding Guidelines",
+			"Issue Tracking with bd",
+			"Essential Commands",
+			"Important Rules",
 		}
 
 		for _, section := range expectedSections {