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/dep.go

@@ -1,3 +1,4 @@
+// Package main implements the bd CLI dependency management commands.
 package main
 
 import (