Add 'last touched' issue tracking for update/close without ID

When no issue ID is provided to `bd update` or `bd close`, use the last
touched issue from the most recent create, update, show, or close operation.

This addresses the common workflow where you create an issue and then
immediately want to add more details (like changing priority from P2 to P4)
without re-typing the issue ID.

Implementation:
- New file last_touched.go with Get/Set/Clear functions
- Store last touched ID in .beads/last-touched (gitignored)
- Track on create, update, show, and close operations
- Allow update/close with zero args to use last touched

(bd-s2t)

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

cmd/bd/create.go

@@ -316,6 +316,9 @@ var createCmd = &cobra.Command{
 				fmt.Printf("  Priority: P%d\n", issue.Priority)
 				fmt.Printf("  Status: %s\n", issue.Status)
 			}
+
+			// Track as last touched issue
+			SetLastTouchedID(issue.ID)
 			return
 		}
 
@@ -514,6 +517,9 @@ var createCmd = &cobra.Command{
 			// Show tip after successful create (direct mode only)
 			maybeShowTip(store)
 		}
+
+		// Track as last touched issue
+		SetLastTouchedID(issue.ID)
 	},
 }