Address remaining golangci-lint findings

- Add package comment to cmd/bd/dep.go
- Change directory permissions from 0755 to 0750 in init.go
- Simplify getNextID signature (remove unused error return)
- Configure golangci-lint exclusions for false positives
- Document linting policy in LINTING.md

The remaining ~100 lint warnings are documented false positives:
- 73 errcheck: deferred cleanup (idiomatic Go)
- 17 revive: Cobra interface requirements and naming choices
- 7 gosec: false positives on validated SQL and user file paths
- 2 dupl: acceptable test code duplication
- 1 goconst: test constant repetition

See LINTING.md for full rationale. Contributors should focus on
avoiding NEW issues rather than the documented baseline.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

cmd/bd/init.go

@@ -30,7 +30,7 @@ and database file. Optionally specify a custom issue prefix.`,
 
 		// Create .beads directory
 		beadsDir := ".beads"
-		if err := os.MkdirAll(beadsDir, 0755); err != nil {
+		if err := os.MkdirAll(beadsDir, 0750); err != nil {
 			fmt.Fprintf(os.Stderr, "Error: failed to create %s directory: %v\n", beadsDir, err)
 			os.Exit(1)
 		}