Add composable issue validators to internal/validation package:
- IssueValidator type with Chain() composition function
- Exists(), NotTemplate(), NotPinned(), NotClosed(), NotHooked() validators
- HasStatus(), HasType() for checking allowed values
- ForUpdate(), ForClose(), ForDelete(), ForReopen() convenience chains
Update cmd/bd/show_unit_helpers.go to use centralized validators instead
of duplicated inline validation logic. This enables consistent validation
across all commands with a single source of truth.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
